Pull clang-cc code for generating PTH files based on the input type.

Pull clang-cc code for generating PTH files based on the input type.
 - <rdar://problem/6741594> [pth] don't abuse -x to drive pth
   generation

 - Simpler, and fixes PR3915.

Cleanup test cases for PTH:
 - Update to use -emit-pth

 - Removed PTH test of carbon.c and cocoa.mm; these didn't actually
   verify anything, and since PTH is token based the extra coverage
   (over cocoa.m) isn't particularly helpful.

 - Split PTH tests in cocoa.m to cocoa-pth.m, solely to increase
   available parallelism when running tests.

Ted, could you update the PTH test cases (include-pth.c and
cocoa-pth.m) to have some sort of positive check that the PTH is
getting used? "# of PTH cache hits" or "tokens read from PTH cache"
statistics would work great. :)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@68189 91177308-0d34-0410-b5e6-96231b3b80d8