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/clangtool.cpp

@@ -717,7 +717,6 @@ void ClangTool::startTool(ClangTool::FileSelection fileSelection,
 
 Diagnostics ClangTool::read(OutputFileFormat outputFileFormat,
                             const QString &logFilePath,
-                            const QString &mainFilePath,
                             const QSet<FilePath> &projectFiles,
                             QString *errorMessage) const
 {
@@ -730,10 +729,8 @@ Diagnostics ClangTool::read(OutputFileFormat outputFileFormat,
                                        acceptFromFilePath,
                                        errorMessage);
     }
-    return readSerializedDiagnostics(Utils::FilePath::fromString(logFilePath),
-                                     Utils::FilePath::fromString(mainFilePath),
-                                     acceptFromFilePath,
-                                     errorMessage);
+
+    return {};
 }
 
 FileInfos ClangTool::collectFileInfos(Project *project, FileSelection fileSelection)