[Driver] Update Fuchsia driver path handling

[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@307830 91177308-0d34-0410-b5e6-96231b3b80d8