Disable LLVM LIT suites and tests
This patch is a little more selective than the clang counterpart. Most
of the LLVM tests are expected to pass in DXC.
In cases where very few tests are failing for obvious reasons I've
disabled individual tests using `REQUIRES` lines. In most places I've
disabled whole suites. Any suite that is expected to pass, I've also
included a comment of the failing test cases so that we can investigate
further.
IR Verifier and Integer tests are disabled only on Windows. These tests
pass on *nix platforms but fail on Windows due to the MS filesystem
modifications causing llvm-as to crash.
ea3c096960