PGO: Rename variables to avoid referring to the "MangledName" of a function.

For C++ functions, we will continue to use the mangled name to identify
functions in the PGO profile data, but this name is confusing for things like
Objective-C methods. For functions with local linkage, we're also going to
include the file name to help distinguish those functions, so this changes to
use more generic variable names.

No functional changes.

llvm-svn: 203074