change FnStubs from being a StringMap<std::string> to being a much

more efficient SmallPtrSet<MCSymbol*>.  This eliminates string
craziness and fixes CodeGen/X86/darwin-quote.ll with the new asmprinter.

Codegen is producing stubs in a nondeterminstic order, but it was doing
this before anyway.

llvm-svn: 81511