IR: Give function GlobalValue::getRealLinkageName() a less misleading name:...

IR: Give function GlobalValue::getRealLinkageName() a less misleading name: dropLLVMManglingEscape().

This function gives the wrong answer on some non-ELF platforms in some
cases. The function that does the right thing lives in Mangler.h. To try to
discourage people from using this function, give it a different name.

Differential Revision: https://reviews.llvm.org/D33162

llvm-svn: 303134