[MS] Mangle a hash of the main file path into anonymous namespaces

Summary:
This is needed to avoid conflicts in mangled names for codeview types in
anonymous namespaces. In CodeView, types refer to each other typically
through forward declarations, which contain mangled names. These names
have to be unique, otherwise the debugger will look up the mangled name
and find the wrong definition.

Furthermore, ThinLTO will deduplicate the types, and debug info
verification can fail when the types have the wrong sizes. This is
PR38608.

Fixes PR38609.

Reviewers: majnemer, inglorion, hans

Subscribers: mehdi_amini, aprantl, JDevlieghere, dexonsmith, cfe-commits

Differential Revision: https://reviews.llvm.org/D50877

llvm-svn: 340079