[lit] Update clang and lld to use new config helpers.

NFC intended here, this only updates clang and lld's lit configs
to use some helper functionality in the lit.llvm submodule.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@313579 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed