[Frontend] Flip default of CreateInvocationOptions::ProbePrecompiled to false

This is generally a better default for tools other than the compiler, which
shouldn't assume a PCH file on disk is something they can consume.

Preserve the old behavior in places associated with libclang/c-index-test
(including ASTUnit) as there are tests relying on it and most important
consumers are out-of-tree. It's unclear whether the tests are specifically
trying to test this functionality, and what the downstream implications of
removing it are. Hopefully someone more familiar can clean this up in future.

Differential Revision: https://reviews.llvm.org/D125149