[index/AST] Determine if a typedef shares a name and spelling location with its under...

[index/AST] Determine if a typedef shares a name and spelling location with its underlying tag type

In such a case, as when using the NS_ENUM macro, for indexing purposes treat the typedef as 'transparent',
meaning we treat its references as symbols of the underlying tag symbol.
Also provide a libclang API to check for such typedefs.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@298392 91177308-0d34-0410-b5e6-96231b3b80d8