add -fpch-codegen/debuginfo mapping to -fmodules-codegen/debuginfo

Using -fmodules-* options for PCHs is a bit confusing, so add -fpch-*
variants. Having extra options also makes it simple to do a configure
check for the feature.
Also document the options in the release notes.

Differential Revision: https://reviews.llvm.org/D83623

(cherry picked from commit 54eea612)