Driver: Add a -stdlib= argument which can be used to select the C++ standard

library to use.
 - This is currently useful for testing libc++; you can now use 'clang++
   -stdlib=libc++ t.cpp' to compile using it if installed.

 - In the future could also be used to select other standard library choices if
   alternatives become available (for example, to use an alternate C library).

llvm-svn: 113891