Change CodeGenModule to rely on the Module's symbol table instead of

shadowing it in the GlobalDeclMap.  Eliminates the string-uniquing
requirement for mangled names, which should help C++ codegen times a little.
Forces us to do string lookups instead of pointer lookups, which might hurt
codegen times a little across the board.  We'll see how it plays out.

Removing the string-uniquing requirement implicitly fixes any bugs like
PR6635 which arose from the fact that we had multiple uniquing tables for
different kinds of identifiers.

llvm-svn: 99012