Fix discovery of clang builtin directory with clang svn versions

Summary:
The CLANG_VERSION for svn clang versions have a svn suffix "8.0.0svn" and all the libraries have a svn suffix.
But not the in builtin include dir. It still points to
"/usr/lib/clang/8.0.0/include/cpuid.h"
without the suffix.

Extend the PATH_SUFFIXES to also looks a version without svn suffix.

BUG: 399439

Test Plan:

• install clang-svn
• execute cmake .. (for kdevelop)

-> it will fail

Reviewers: KDevelop, kfunk

Reviewed By: KDevelop, kfunk

Subscribers: kdevelop-devel

Tags: KDevelop

Differential Revision: https://phabricator.kde.org/D15998