[analyzer] Teach scan-build to find clang when installed in /usr/local/bin/
Change scan-build to support the scenario where scan-build is installed in
$TOOLCHAIN/usr/local/bin/ but clang itself is installed in $TOOLCHAIN/usr/bin/.
This is restricted to when 'xcrun' is present; that is, on the Mac.
Differential Revision: https://reviews.llvm.org/D59406