Add AST_SIGNATURE record to unhashed control block of PCM files

Summary:
This record is constructed by hashing the bytes of the AST block in a similiar
fashion to the SIGNATURE record. This new signature only means anything if the
AST block is fully relocatable, i.e. it does not embed absolute offsets within
the PCM file. This change ensure this does not happen by replacing these offsets
with offsets relative to the nearest relevant subblock of the AST block.

Reviewers: Bigcheese, dexonsmith

Subscribers: dexonsmith, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D80383