[Docs] Add Documentation on (Thin)LTO + PGO Build Configs

This patch adds documentation on the AdvancedBuilds page on how to do
PGO builds with (Thin)LTO with the currently undocumented (as far as I
can tell) PGO_INSTRUMENT_LTO option in the Clang PGO caches.

Reviewed By: sylvestre.ledru

Differential Revision: https://reviews.llvm.org/D137898