FindPyModGen: Also search for an unversioned clang++

Summary:
While my distro does have a versioned clang executable, it doesn't
have a versioned clang++ executable. The versioned executable is
still searched first, falling back to the unversioned one.

Reviewers: Frameworks, Build System

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D5291