[NFC] Remove IRGen's ClangTypeConverter in favor of the AST one.

include/swift/AST/ASTContext.h

@@ -640,6 +640,14 @@ class ASTContext final {
   /// if applicable.
   const Decl *getSwiftDeclForExportedClangDecl(const clang::Decl *decl);
 
+  /// General conversion method from Swift types -> Clang types.
+  ///
+  /// HACK: This method is only intended to be called from a specific place in
+  /// IRGen. For converting function types, strongly prefer using one of the
+  /// other methods instead, instead of manually iterating over parameters
+  /// and results.
+  const clang::Type *getClangTypeForIRGen(Type ty);
+
   /// Determine whether the given Swift type is representable in a
   /// given foreign language.
   ForeignRepresentationInfo

lib/AST/ASTContext.cpp

@@ -54,6 +54,7 @@
 #include "swift/Subsystems.h"
 #include "swift/Syntax/References.h"
 #include "swift/Syntax/SyntaxArena.h"
+#include "clang/AST/Type.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/ADT/StringMap.h"
@@ -4589,6 +4590,10 @@ ASTContext::getSwiftDeclForExportedClangDecl(const clang::Decl *decl) {
   return impl.Converter->getSwiftDeclForExportedClangDecl(decl);
 }
 
+const clang::Type *
+ASTContext::getClangTypeForIRGen(Type ty) {
+  return getClangTypeConverter().convert(ty).getTypePtrOrNull();
+}
 
 CanGenericSignature ASTContext::getSingleGenericParameterSignature() const {
   if (auto theSig = getImpl().SingleGenericParameterSignature)

lib/AST/ClangTypeConverter.cpp

@@ -12,13 +12,14 @@
 //
 // This file implements generation of Clang AST types from Swift AST types for
 // types that are representable in Objective-C interfaces.
-// Large chunks of the code are lightly modified versions of the code in
-// IRGen/GenClangType.cpp (which should eventually go away), so make sure
-// to keep the two in sync.
-// The three major differences are that, in this file:
+//
+// The usage of ClangTypeConverter at the AST level means that we may
+// encounter ill-formed types and/or sugared types. To avoid crashing and
+// keeping sugar as much as possible (in case the generated Clang type needs
+// to be surfaced to the user):
+//
 // 1. We fail gracefully instead of asserting/UB.
 // 2. We try to keep clang sugar instead of discarding it.
-// 3. We use getAs instead of cast as we handle Swift types with sugar.
 //
 //===----------------------------------------------------------------------===//
 

lib/AST/ClangTypeConverter.h

@@ -94,6 +94,8 @@ class ClangTypeConverter :
       SmallVectorImpl<clang::TemplateArgument> &templateArgs);
 
 private:
+  friend ASTContext; // HACK: expose `convert` method to ASTContext
+
   clang::QualType convert(Type type);
 
   clang::QualType convertMemberType(NominalTypeDecl *DC,