[clang][modules] Use relative offsets for input files

This patch replaces absolute offsets into the input files block with offsets relative to the block start. This makes the whole section "relocatable". I confirmed all other uses of `GetCurrentBitNo()` are turned into relative offsets before being serialized into the AST file.

Reviewed By: benlangmuir

Differential Revision: https://reviews.llvm.org/D158572