Move all of the GoogleTest files back to the same locations they occupy

Move all of the GoogleTest files back to the same locations they occupy
externally to simplify our integration of GoogleTest into LLVM. Also,
build the single source file gtest-all.cc instead of the individual
source files as we don't expect these to change and thus gain nothing
from increased incrementality in compiles.

This makes our standard build of googletest exactly like upstream's
recommended build and the sanitizer's build. It also simplifies the
steps of importing a new version should we ever want one.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194801 91177308-0d34-0410-b5e6-96231b3b80d8