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

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.

This disables the failing runtime test on Windows, since the compiler
options (-fPIC) are unsupported on that platform.

Reviewed By: phosek

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