Reland "[pgo] Avoid introducing relocations by using private alias""

In many cases, we can use an alias to avoid a symbolic relocations,
instead of using the public, interposable symbol. When the instrumented
function is in a COMDAT, we can use a hidden alias, and still avoid
references to discarded sections.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D137982