Improve CMake and LIT support for Windows

This patch contains multiple cleanups and fixes to better support building on
Windows.

* [Test] Fix handling of library runtime search paths by correctly adding them
  to the PATH variable when running the tests.

* [Test] Don't explicitly force "--target=i686-pc-windows" when running the
  test suite. Clang++ seems to deduce the correct target.

* [Test] Fix `.sh.cpp` tests on Windows by properly escaping flags used in
  shell commands. Specifically windows style paths which included spaces
  were causing these tests to fail.

* [CMake] Add "vcruntime" to the list of supported C++ ABI libraries in CMake, and
  teach the test suite how to handle it. For now libc++ defaults to using
  "vcruntime" on Windows except when libc++abi is in tree; That is probably
  a bug and should be changed to always use vcruntime, at least for now.

* [Misc] Move the "c++-build" include directory to the libc++ binary dir
  instead of the top level project dir and rename it "c++build". This is just
  misc cleanup. Libc++ shouldn't be creating internal build files and directories
  at the top-level projects root.

* [Misc] Build type_info's destructor when building for MSVC. This is a temporary
  work around to prevent link errors until we have a proper type_info
  implementation.

llvm-svn: 292157