scan-build: Set CC and CXX as make variables when wrapping make builds.
scan-build: Set CC and CXX as make variables when wrapping make builds.

Variables set in a makefile are not overridden by environment variables.
Make sure we actually override CC and CXX when using scan-build.

Patch by Steve McCoy!

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@189372 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang