llvm_gcc_supports function that takes the language as an argument. Base that
function on the new LLVMGCC_LANGS configured variable so that we don't have
to execute feature checks during the test run.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@36322 91177308-0d34-0410-b5e6-96231b3b80d8