Forward WrapperFrontendAction::shouldEraseOutputFiles()

Per the documentation, this class is supposed to forward every virtual
method, but we had missed on (shouldEraseOutputFiles). This fixes using
a wrapped frontend action over the PCH generator when using
-fallow-pch-with-compiler-errors. I do not think any upstream wrapper
actions can test this.

Differential Revision: https://reviews.llvm.org/D77180

rdar://61110294