Enable and require C++17 compilation

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/+/51948
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
diff --git a/Android.bp b/Android.bp
index 248a1c6..94c173e 100644
--- a/Android.bp
+++ b/Android.bp
@@ -32,7 +32,7 @@
         "-DVK_USE_PLATFORM_ANDROID_KHR",
         "-DVK_EXPORT= ",
     ],
-    cpp_std: "c++14",
+    cpp_std: "c++17",
 
     target: {
         host: {