[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.

rdar://problem/48914634

Differential Revision: https://reviews.llvm.org/D59406

llvm-svn: 356308