[clang-format] Fix bug where -dump-config failed on ObjC header

Summary:
`clang-format -dump-config path/to/file.h` never passed
anything for the Code parameter to clang::format::getStyle().

This meant the logic to guess Objective-C from the contents
of a .h file never worked, because LibFormat didn't have the
code to work with.

With this fix, we now correctly read in the contents of the
file if possible with -dump-config.

I had to update the lit config for test/Format/ because
the default config ignores .h files.

Test Plan: make -j12 check-clang

Reviewers: jolesiak, krasimir

Reviewed By: jolesiak, krasimir

Subscribers: Wizard, klimek, cfe-commits, djasper

Differential Revision: https://reviews.llvm.org/D42395

llvm-svn: 323668