[analyzer] Improved behavior if Clang was not found, part II

- scan-build help: display 'Could not query Clang for the list of available checkers.' + the reason why it happened so if clang was not found.
- display requested/forced help in case of --use-analyzer=Xcode.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@247828 91177308-0d34-0410-b5e6-96231b3b80d8