[analyzer] Improve behavior if Clang not found.
[analyzer] Improve behavior if Clang not found.

- Eliminate 'No such file or directory at scan-build line ...' error if '$RealBin/bin/clang' or '$RealBin/clang' directory does not exist.
- Eliminate 'Use of uninitialized value $Clang in concatenation (.) or string at scan-build line ...' error if help is displayed while $Clang was not found.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@247466 91177308-0d34-0410-b5e6-96231b3b80d8

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