[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.

llvm-svn: 247466