Re-applies the patch first applied way back in r106099, with

Re-applies the patch first applied way back in r106099, with
accompanying fixes to make it work today.

The core of this patch is to provide a link from a TemplateTypeParmType
back to the TemplateTypeParmDecl node which declared it. This in turn
provides much more precise information about the type, where it came
from, and how it functions for AST consumers.

To make the patch work almost a year after its first attempt, it needed
serialization support, and it now retains the old getName() interface.
Finally, it requires us to not attempt to instantiate the type in an
unsupported friend decl -- specifically those coming from template
friend decls but which refer to a specific type through a dependent
name.

A cleaner representation of the last item would be to build
FriendTemplateDecl nodes for these, storing their template parameters
etc, and to perform proper instantation of them like any other template
declaration. They can still be flagged as unsupported for the purpose of
access checking, etc.

This passed an asserts-enabled bootstrap for me, and the reduced test
case mentioned in the original review thread no longer causes issues,
likely fixed at somewhere amidst the 24k revisions that have elapsed.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@130628 91177308-0d34-0410-b5e6-96231b3b80d8