ClangStaticAnalyzer: Fix accepting "clang" executable

The user can set "clang.exe" in the options, which is fine. However, "clang"
was problematic on Windows since we failed to replace it with "clang-cl" for
the actual run for MSVC toolchains.

Change-Id: I2f96428d15c7406e90a38c797817aef34e7a9541
Reviewed-by: Christian Kandeler <christian.kandeler@theqtcompany.com>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>