[Frontend] Make WrapperFrontendAction call WrappedAction.PrepareToExecuteAction.

Fixes `-emit-header-module` when GenerateHeaderModuleAction is wrapped
by another frontend action.

rdar://problem/47302588

Reviewers: rsmith, arphaman

Reviewed By: arphaman

Subscribers: jkorous, dexonsmith, cfe-commits

Differential Revision: https://reviews.llvm.org/D56766

llvm-svn: 351402