[CodeGen] Add additional mangling for struct members of non trivial structs

In https://bugs.llvm.org/show_bug.cgi?id=41206 we observe bad codegen
when embedding a non-trivial C struct within a C struct. This is due to
the fact that name mangling for non-trivial structs marks the two
structs as identical. This diff contains a fix for this issue.

Patch by Dan Zimmerman <daniel.zimmerman@me.com>.

Differential Revision: https://reviews.llvm.org/D59873

llvm-svn: 357184