[modules] Allow to validate system headers less often with...

[modules] Allow to validate system headers less often with `-fmodules-validate-once-per-build-session`.

Make flags `-fmodules-validate-system-headers` and
`-fmodules-validate-once-per-build-session` orthogonal, so they have
their own independent responsibilities - if system headers should be
validated and how often.

rdar://87988889

Differential Revision: https://reviews.llvm.org/D135232