This patch does two seperate things. First it adds a file called
"__libcpp_version" which only contains the current libc++ version
(currently 4000). This file is not intended for use as a header. This file
is used by Clang in order to easily determine the installed libc++ version.
This allows Clang to enable/disable certain language features only when the
library supports them.
The second change is the addition of _LIBCPP_LIBRARY_VERSION macro, which
returns the version of the installed dylib since it may be different than
the headers.
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@285382 91177308-0d34-0410-b5e6-96231b3b80d8
