[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