Updates build files to specify using the C++17 standard.

Add a usage of the C++17 exclusive [[maybe_unused]] attribute as a test.
Also unconditionally use <filesystem> in Reactor unit tests.

C++17 deprecates things like std::unary_function<>, which were present
in Subzero's LLVM headers, but the functionality was unused so it has
been deleted.

Bug: b/174843857
Bug: b/155971541
Change-Id: Ib167726535399b0b12751a3d93feaa9db762cd5d
Signed-off-by: Nicolas Capens <capn@google.com>
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/51948Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>