[NativePDB] Correctly reconstruct DeclContext for nested enums.

We reconstruct the AST hierarchy by trying to hack up a mangled
name for the parent type using the child type's mangled name.
This was failing for enums because their tag type is represented
with two letters ("W4") instead of one letter ("T", "U", etc) as
it is with classes, structs, and unions.  After accounting for
this we can now correctly determine when an enum is nested
inside of a namespace or a class.

llvm-svn: 349565