ClangTools: Remove dependency to libclang and custom clang binary

Before this change, we've invoked a custom clang binary that had clazy
statically compiled into it. The invocation also ensured that the
diagnostics were serialized to a file, so that libclang could be used
afterwards to read them.

As the clazy-standalone executable supports exporting diagnostics to a
YAML file now (just as clang-tidy) and Qt Creator ships it already, rely
on that executable alone instead of the clang/libclang combo.

While we do not depend on any clang header or library at build-time now,
the CompilerOptionsBuilder constructor still needs the CLANG_VERSION and
CLANG_RESOURCE_DIR pieces from llvm-config. This dependency should be
removed as next.

Change-Id: I4fa5753ab09008fd24bc5247b28c4836b5e8ca45
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>
Reviewed-by: hjk <hjk@qt.io>

src/plugins/clangtools/clangtoolsutils.cpp

@@ -153,7 +153,6 @@ QString clazyStandaloneFallbackExecutable()
 {
     return findValidExecutable({
         shippedClazyStandaloneExecutable(),
-        qEnvironmentVariable("QTC_USE_CLAZY_STANDALONE_PATH"),
         Constants::CLAZY_STANDALONE_EXECUTABLE_NAME,
     });
 }