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
57f7dafcb7