[nfc] [cxx-interop] Move function template specialization logic into clang-module-loader.

Author: zoecarver

include/swift/AST/ClangModuleLoader.h

@@ -31,10 +31,12 @@ class Type;
 
 namespace swift {
 
+class ConcreteDeclRef;
 class Decl;
 class DeclContext;
 class EffectiveClangContext;
 class SwiftLookupTable;
+class ValueDecl;
 class VisibleDeclConsumer;
 
 /// Represents the different namespaces for types in C.
@@ -201,6 +203,10 @@ class ClangModuleLoader : public ModuleLoader {
   instantiateCXXClassTemplate(clang::ClassTemplateDecl *decl,
                       ArrayRef<clang::TemplateArgument> arguments) = 0;
 
+  virtual ConcreteDeclRef
+  getCXXFunctionTemplateSpecialization(SubstitutionMap subst,
+                                       ValueDecl *decl) = 0;
+
   /// Try to parse the string as a Clang function type.
   ///
   /// Returns null if there was a parsing failure.

include/swift/ClangImporter/ClangImporter.h

@@ -61,17 +61,20 @@ class CompilerInvocation;
 class ClangImporterOptions;
 class ClangModuleUnit;
 class ClangNode;
+class ConcreteDeclRef;
 class Decl;
 class DeclContext;
 class EffectiveClangContext;
 class EnumDecl;
+class FuncDecl;
 class ImportDecl;
 class IRGenOptions;
 class ModuleDecl;
 class NominalTypeDecl;
 class StructDecl;
 class SwiftLookupTable;
 class TypeDecl;
+class ValueDecl;
 class VisibleDeclConsumer;
 enum class SelectorSplitKind;
 
@@ -261,6 +264,9 @@ class ClangImporter final : public ClangModuleLoader {
   instantiateCXXClassTemplate(clang::ClassTemplateDecl *decl,
                       ArrayRef<clang::TemplateArgument> arguments) override;
 
+  ConcreteDeclRef getCXXFunctionTemplateSpecialization(
+          SubstitutionMap subst, ValueDecl *decl) override;
+
   /// Just like Decl::getClangNode() except we look through to the 'Code'
   /// enum of an error wrapper struct.
   ClangNode getEffectiveClangNode(const Decl *decl) const;