Fix -Wnonportable-include-path suppression for header maps with absolute paths.

In `DirectoryLookup::LookupFile` parameter `HasBeenMapped` doesn't cover
the case when clang finds a file through a header map but doesn't remap
the lookup filename because the target path is an absolute path. As a
result, -Wnonportable-include-path suppression for header maps
introduced in r301592 wasn't triggered.

Change parameter `HasBeenMapped` to `IsInHeaderMap` and use parameter
`MappedName` to track the filename remapping. This way we can handle
both relative and absolute paths in header maps, and account for their
specific properties, like filename remapping being a property preserved
across lookups in multiple directories.

rdar://problem/39516483

Reviewers: dexonsmith, bruno

Reviewed By: dexonsmith

Subscribers: jkorous, cfe-commits, ributzka

Differential Revision: https://reviews.llvm.org/D58094

llvm-svn: 371655