[libc++] Populate a lit feature including the compiler patch level

If the compiler is (for example) AppleClang 10.0.1, we would previously
populate the following lit features:

    apple-clang
    apple-clang-10
    apple-clang-10.0

This patch additionally populates a feature called 'apple-clang-10.0.1',
which allows more precise enabling/disabling of tests.

llvm-svn: 369406