[pch] Honour -fallow-pch-with-compiler-errors for overall compilation status

Previously we would emit a PCH with errors, but fail the overall
compilation. If run using the driver, that would result in removing the
just-produced PCH. Instead, we should have the compilation result match
whether we were able to emit the PCH.

Differential Revision: https://reviews.llvm.org/D77159

rdar://61110294