pass environment when invoking llvm-config and clang from lit.cfg

Use the same environment when invoking llvm-config from lit.cfg as
will be used when running tests, so that ASAN_OPTIONS, INCLUDE, etc.
are present.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@218404 91177308-0d34-0410-b5e6-96231b3b80d8