Two changes to scan-build:

(1) When no 'clang' is found with 'scan-build', remember the one from
the path as scan-build sees it, not the build system.  This prevents
us from finding different clangs during the build.

(2) Don't set LDPLUSPLUS when running xcodebuild; instead rely on the
clang driver to do the right thing.

llvm-svn: 95943