[CMake] Don't cache LLVM_MAIN_SRC_DIR

If we're not in a standalone build, this variable should be already
set, so there's no need to set it again or to cache it.

Differential Revision: https://reviews.llvm.org/D57993

git-svn-id: https://llvm.org/svn/llvm-project/libunwind/trunk@353915 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed