check-clang: Introduce get_llvm_config_props in clang/test/lit.cfg.

In trunk, we can use features as below:

  aarch64-registered-target
  hexagon-registered-target
  msp430-registered-target
  r600-registered-target
  systemz-registered-target
  xcore-registered-target

Each of them, as below, implies corresponding subtargets:

  arm-registered-target   -- arm, thumb
  mips-registered-target  -- mips, mips64, mips64el, mipsel
  nvptx-registered-target -- nvptx, nvptx64
  sparc-registered-target -- sparc, sparcv9
  x86-registered-target   -- x86, x86-64

They will be renamed:

  cppbackend-registered-target -- was "cpp". Unused in trunk.
  powerpc-registered-target -- was "ppc32", "ppc64" and "ppc64le".

The feature "asserts" is also taken from llvm-config.

llvm-svn: 196347