Rather than rejecting attempts to run preprocessor-only actions on AST files,

Rather than rejecting attempts to run preprocessor-only actions on AST files,
replay the steps taken to create the AST file with the preprocessor-only action
installed to produce preprocessed output.

This can be used to produce the preprocessed text for an existing .pch or .pcm
file.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@304726 91177308-0d34-0410-b5e6-96231b3b80d8