Modify the LLVM checkout of googletest:

Modify the LLVM checkout of googletest:
1) don't delete gtest-all.cc (which is used to gather all gtest source
   files in a single file)
2) make including LLVMSupport headers optional (on by default).
   Sanitizer tools may want to use their own versions of googletest
   compiled with specific flags, instead of the common googletest
   library used for all other LLVM/Clang unittests.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@169559 91177308-0d34-0410-b5e6-96231b3b80d8