Re-land parsing and printing for Clang function types.

This reverts commit e805fe486e, which reverted
the change earlier. The problem was caused due to a simultaneous change to some
code by the PR with parsing and printing for Clang function types (#28737)
and the PR which introduced Located<T> (#28643).

This commit also includes a small change to make sure the intersecting region
is fixed: the change is limited to using the fields of Located<T> in the
`tryParseClangType` lambda.

include/swift/Frontend/ModuleInterfaceSupport.h

@@ -31,6 +31,10 @@ struct ModuleInterfaceOptions {
   /// interface, or should we fully-qualify them?
   bool PreserveTypesAsWritten = false;
 
+  /// Should we emit the cType when printing @convention(c) or no?
+  /// FIXME: [clang-function-type-serialization] This check should go away.
+  bool PrintFullConvention = false;
+
   /// Copy of all the command-line flags passed at .swiftinterface
   /// generation time, re-applied to CompilerInvocation when reading
   /// back .swiftinterface and reconstructing .swiftmodule.