Fix expression parser to not accept any type whose basename matches for a type...

Fix expression parser to not accept any type whose basename matches for a type that must exist at root level

This patch fixes an issue where we weren't looking for exact matches in the expression parser and also fixed the type lookup logic in the Module.cpp. Tests added to make sure we don't regress.

Differential Revision: https://reviews.llvm.org/D46128

llvm-svn: 331227