Move doc comment to header file

Author: j-hui

lib/AST/ClangTypeConverter.cpp

@@ -905,9 +905,6 @@ Decl *ClangTypeConverter::getSwiftDeclForExportedClangDecl(
 }
 
 clang::QualType ClangTypeConverter::convertTemplateArgument(Type type) {
-  // C++ function templates can only be instantiated with Clang types and
-  // a handful of Swift builtin types. These are enumerated here rather than
-  // delegated to ClangTypeConverter::convert() (which is more general).
   auto withCache = [&](auto lookup) {
     auto [it, inserted] = Cache.try_emplace(type, clang::QualType{});
     if (!inserted)

lib/AST/ClangTypeConverter.h

@@ -116,6 +116,11 @@ class ClangTypeConverter :
   clang::QualType convertMemberType(NominalTypeDecl *DC,
                                     StringRef memberName);
 
+  /// Convert Swift types that are used as C++ function template arguments.
+  ///
+  /// C++ function templates can only be instantiated with types originally
+  /// imported from Clang, and a handful of builtin Swift types (e.g., integers
+  /// and floats).
   clang::QualType convertTemplateArgument(Type type);
 
   clang::QualType convertClangDecl(Type type, const clang::Decl *decl);