[scan-build] Fix clang++ pathname again

e00629f7 "[scan-build] Fix clang++ pathname" had
removed the -MAJOR.MINOR suffix, but since presumably LLVM 7 the suffix is only
-MAJOR, so ClangCXX (i.e., the CLANG_CXX environment variable passed to
clang/tools/scan-build/libexec/ccc-analyzer) now contained a non-existing
/path/to/clang-12++ (which apparently went largely unnoticed as
clang/tools/scan-build/libexec/ccc-analyzer falls back to just 'clang++' if the
executable denoted by CLANG_CXX does not exist).

For the new clang/test/Analysis/scan-build/cxx-name.test to be effective,
%scan-build must now take care to pass the clang executable's resolved pathname
(i.e., ending in .../clang-MAJOR rather than just .../clang) to --use-analyzer.

Differential Revision: https://reviews.llvm.org/D89481