[analyzer] Teach scan-build how to pass -analyzer-config to xcodebuild.

The scan-build tool assists various build systems with applying the Clang
static analyzer alongside compilation. It offers explicit integration with
Xcode's native build system aka `xcodebuild`; in this case it doesn't
substitute the compiler, but instead kindly asks xcodebuild to enable
the static analyzer, something that it already knows how to do.

Make sure scan-build's `-analyzer-config` flag (which translates to a
similar `clang -cc1 -analyzer-config` flag) is properly translated
to Xcode build system. This unbreaks a few related features such as
checker silencing.

No LIT tests because they'd require an Xcode installation on your system.