[-gmodules] Honor -fdebug-prefix-map in the debug info inside PCMs.

This patch passes -fdebug-prefix-map (a feature for renaming source
paths in the debug info) through to the per-module codegen options and
adds the debug prefix map to the module hash.

<rdar://problem/46045865>

Differential Revision: https://reviews.llvm.org/D55037

llvm-svn: 347926