[ClangImporter] Allow importing without a resource dir on non-Darwin

Previously, we would assert that there's a runtime resource dir
available, but then accept the possibility of no glibc.modulemap. We
should just do the same thing for 'no resource dir' as 'no module map'.

Author: Harlan Haskins

lib/ClangImporter/ClangImporter.cpp

@@ -407,6 +407,35 @@ void ClangImporter::clearTypeResolver() {
 
 #pragma mark Module loading
 
+/// Finds the glibc.modulemap file relative to the provided resource dir.
+///
+/// Note that the module map used for Glibc depends on the target we're
+/// compiling for, and is not included in the resource directory with the other
+/// implicit module maps. It's at {freebsd|linux}/{arch}/glibc.modulemap.
+static Optional<StringRef>
+getGlibcModuleMapPath(StringRef resourceDir, llvm::Triple triple,
+                      SmallVectorImpl<char> &scratch) {
+  if (resourceDir.empty())
+    return None;
+
+  scratch.append(resourceDir.begin(), resourceDir.end());
+  llvm::sys::path::append(
+      scratch,
+      swift::getPlatformNameForTriple(triple),
+      swift::getMajorArchitectureName(triple),
+      "glibc.modulemap");
+
+  // Only specify the module map if that file actually exists.
+  // It may not--for example in the case that
+  // `swiftc -target x86_64-unknown-linux-gnu -emit-ir` is invoked using
+  // a Swift compiler not built for Linux targets.
+  if (llvm::sys::fs::exists(scratch)) {
+    return StringRef(scratch.data(), scratch.size());
+  } else {
+    return None;
+  }
+}
+
 static void
 getNormalInvocationArguments(std::vector<std::string> &invocationArgStrs,
                              ASTContext &ctx,
@@ -571,28 +600,10 @@ getNormalInvocationArguments(std::vector<std::string> &invocationArgStrs,
       }
     }
 
-    // The module map used for Glibc depends on the target we're compiling for,
-    // and is not included in the resource directory with the other implicit
-    // module maps. It's at {freebsd|linux}/{arch}/glibc.modulemap.
     SmallString<128> GlibcModuleMapPath;
-    GlibcModuleMapPath = searchPathOpts.RuntimeResourcePath;
-
-    // Running without a resource directory is not a supported configuration.
-    assert(!GlibcModuleMapPath.empty());
-
-    llvm::sys::path::append(
-      GlibcModuleMapPath,
-      swift::getPlatformNameForTriple(triple),
-      swift::getMajorArchitectureName(triple),
-      "glibc.modulemap");
-
-    // Only specify the module map if that file actually exists.
-    // It may not--for example in the case that
-    // `swiftc -target x86_64-unknown-linux-gnu -emit-ir` is invoked using
-    // a Swift compiler not built for Linux targets.
-    if (llvm::sys::fs::exists(GlibcModuleMapPath)) {
-      invocationArgStrs.push_back(
-        (Twine("-fmodule-map-file=") + GlibcModuleMapPath).str());
+    if (auto path = getGlibcModuleMapPath(searchPathOpts.RuntimeResourcePath,
+                                          triple, GlibcModuleMapPath)) {
+      invocationArgStrs.push_back((Twine("-fmodule-map-file=") + *path).str());
     } else {
       // FIXME: Emit a warning of some kind.
     }