In some cases (for example, the Firefox build system), the CLANG_CXX variable is defined but empty.
Extend the test (like it is done in scan-build) to check also if the variable 
is empty or not.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201586 91177308-0d34-0410-b5e6-96231b3b80d8