[Driver] Update Fuchsia driver path handling
Several improvements to the Fuchsia driver:
* Search for C++ library headers and libraries in directories that
are part of the toolchain distribution rather than sysroot.
* Use LLVM support utlities to construct paths to make sure the driver
is also usable on Windows for cross-compiling.
* Change the driver to inherit directly from ToolChain rather than
Generic_GCC since we don't need any of the GCC related multilib logic.
Differential Revision: https://reviews.llvm.org/D32613
llvm-svn: 307830