getMangledTypeStr: clarify how it mangles types, and add tests

"Write a set of tests that show how name mangling is done for overloaded intrinsics."  These happen to use gc.relocates to exercise the codepath in question, but is not a GC specific test.

Patch by: artagnon@gmail.com
Differential Revision: http://reviews.llvm.org/D6915

llvm-svn: 226056