Honor -fdebug-prefix-map when creating function names for the debug info.

Honor -fdebug-prefix-map when creating function names for the debug info.

This adds a callback to PrintingPolicy to allow CGDebugInfo to remap
file paths according to -fdebug-prefix-map. Otherwise the debug info
(particularly function names for C++ lambdas) may contain paths that
should have been remapped in the debug info.

<rdar://problem/46128056>

Differential Revision: https://reviews.llvm.org/D55137

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@348060 91177308-0d34-0410-b5e6-96231b3b80d8