This is a reland of D32613 which fixes the test paths to work on Windows.
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.