[cxx-interop] Recognize CxxStdlib as an alias for std

C++ standard library module is called `std`. To make it more clear to a Swift developer that this module is a C++ stdlib, and not a Swift stdlib, let's rename it to `CxxStdlib`.

This is the first step in the module rename. We don't ban `std` in this patch to be able to build SwiftCompilerSources with hosttools until a new Swift compiler is shipped.

Author: egorzhdan

include/swift/AST/KnownIdentifiers.def

@@ -60,6 +60,7 @@ IDENTIFIER(CoreFoundation)
 IDENTIFIER(count)
 IDENTIFIER(CVarArg)
 IDENTIFIER(Cxx)
+IDENTIFIER(CxxStdlib)
 IDENTIFIER(Darwin)
 IDENTIFIER(Distributed)
 IDENTIFIER(dealloc)

lib/ClangImporter/ClangImporter.cpp

@@ -2001,6 +2001,14 @@ ClangImporter::loadModule(SourceLoc importLoc,
 ModuleDecl *ClangImporter::Implementation::loadModule(
     SourceLoc importLoc, ImportPath::Module path) {
   ModuleDecl *MD = nullptr;
+  ASTContext &ctx = getNameImporter().getContext();
+
+  if (path.front().Item == ctx.Id_CxxStdlib) {
+    ImportPath::Builder adjustedPath(ctx.getIdentifier("std"), importLoc);
+    adjustedPath.append(path.getSubmodulePath());
+    path = adjustedPath.get().getModulePath(ImportKind::Module);
+  }
+
   if (!DisableSourceImport)
     MD = loadModuleClang(importLoc, path);
   if (!MD)

test/Interop/Cxx/stdlib/libcxx-module-interface.swift

@@ -1,6 +1,6 @@
-// RUN: %target-swift-ide-test -print-module -module-to-print=std -source-filename=x -enable-experimental-cxx-interop -tools-directory=%llvm_obj_root/bin -module-cache-path %t | %FileCheck %s  -check-prefix=CHECK-STD
-// RUN: %target-swift-ide-test -print-module -module-to-print=std.iosfwd -source-filename=x -enable-experimental-cxx-interop -tools-directory=%llvm_obj_root/bin -module-cache-path %t | %FileCheck %s  -check-prefix=CHECK-IOSFWD
-// RUN: %target-swift-ide-test -print-module -module-to-print=std.string -source-filename=x -enable-experimental-cxx-interop -tools-directory=%llvm_obj_root/bin -module-cache-path %t | %FileCheck %s  -check-prefix=CHECK-STRING
+// RUN: %target-swift-ide-test -print-module -module-to-print=CxxStdlib -source-filename=x -enable-experimental-cxx-interop -tools-directory=%llvm_obj_root/bin -module-cache-path %t | %FileCheck %s  -check-prefix=CHECK-STD
+// RUN: %target-swift-ide-test -print-module -module-to-print=CxxStdlib.iosfwd -source-filename=x -enable-experimental-cxx-interop -tools-directory=%llvm_obj_root/bin -module-cache-path %t | %FileCheck %s  -check-prefix=CHECK-IOSFWD
+// RUN: %target-swift-ide-test -print-module -module-to-print=CxxStdlib.string -source-filename=x -enable-experimental-cxx-interop -tools-directory=%llvm_obj_root/bin -module-cache-path %t | %FileCheck %s  -check-prefix=CHECK-STRING
 
 // This test is specific to libc++ and therefore only runs on Darwin platforms.
 // REQUIRES: OS=macosx || OS=ios

test/Interop/Cxx/stdlib/libstdcxx-module-interface.swift

@@ -1,5 +1,5 @@
 // RUN: %empty-directory(%t)
-// RUN: %target-swift-ide-test -print-module -module-to-print=std -source-filename=x -enable-experimental-cxx-interop -module-cache-path %t > %t/interface.swift
+// RUN: %target-swift-ide-test -print-module -module-to-print=CxxStdlib -source-filename=x -enable-experimental-cxx-interop -module-cache-path %t > %t/interface.swift
 // RUN: %FileCheck %s -check-prefix=CHECK-STD < %t/interface.swift
 // RUN: %FileCheck %s -check-prefix=CHECK-SIZE-T < %t/interface.swift
 // RUN: %FileCheck %s -check-prefix=CHECK-TO-STRING < %t/interface.swift

test/Interop/Cxx/stdlib/msvcprt-module-interface.swift

@@ -1,6 +1,6 @@
 // RUN: %empty-directory(%t)
-// RUN: %target-swift-ide-test -print-module -module-to-print std -source-filename none -enable-experimental-cxx-interop -module-cache-path %t | %FileCheck %s -check-prefix CHECK-STD
-// RUN: %target-swift-ide-test -print-module -module-to-print=std.string -source-filename=x -enable-experimental-cxx-interop -module-cache-path %t | %FileCheck %s -check-prefix CHECK-STRING
+// RUN: %target-swift-ide-test -print-module -module-to-print CxxStdlib -source-filename none -enable-experimental-cxx-interop -module-cache-path %t | %FileCheck %s -check-prefix CHECK-STD
+// RUN: %target-swift-ide-test -print-module -module-to-print=CxxStdlib.string -source-filename=x -enable-experimental-cxx-interop -module-cache-path %t | %FileCheck %s -check-prefix CHECK-STRING
 
 // This test is specific to msvcprt and therefore only runs on Windows.
 // REQUIRES: OS=windows-msvc

test/Interop/Cxx/stdlib/use-std-string.swift

@@ -1,4 +1,5 @@
 // RUN: %target-run-simple-swift(-I %S/Inputs -Xfrontend -enable-experimental-cxx-interop)
+// RUN: %target-run-simple-swift(-D USE_CXXSTDLIB_SPELLING -I %S/Inputs -Xfrontend -enable-experimental-cxx-interop)
 //
 // REQUIRES: executable_test
 //
@@ -8,11 +9,19 @@
 import StdlibUnittest
 import StdString
 #if os(Linux)
+#if USE_CXXSTDLIB_SPELLING
+import CxxStdlib
+#else
 import std
-// FIXME: import std.string once libstdc++ is split into submodules.
+#endif
+// FIXME: import CxxStdlib.string once libstdc++ is split into submodules.
+#else
+#if USE_CXXSTDLIB_SPELLING
+import CxxStdlib.string
 #else
 import std.string
 #endif
+#endif
 
 var StdStringTestSuite = TestSuite("StdString")