[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

llvm-svn: 353915
GitOrigin-RevId: d2f3882def8a828dfbec3bde517fbef971c5eb92
1 file changed