Re-land: Add Clang shared library with C++ exports

Summary:
This patch adds a libClang_shared library on *nix systems which exports the entire C++ API. In order to support this on Windows we should really refactor llvm-shlib and share code between the two.

This also uses a slightly different method for generating the shared library, which I should back-port to llvm-shlib. Instead of linking the static archives and passing linker flags to force loading the whole libraries, this patch creates object libraries for every library (which has no cost in the build system), and link the object libraries.

llvm-svn: 360985