[libc++] Clarify the name of Lit features related to standard library selection

Before this patch, we had features named 'libc++', 'libstdc++' and
'msvc' to describe the three implementations that use our test suite.
This patch renames them to 'stdlib=libc++', 'stdlib=libstdc++', etc
to avoid confusion between MSVC's STL and the MSVC compiler (or Clang
in MSVC mode).

Furthermore, this prepares the terrain for adding support for additional
"implementations" to the test suite. Basically, I'd like to be able to
treat Apple's libc++ differently from LLVM's libc++ for the purpose of
testing, because those effectively behave in different ways in some aspects.