[C++] Compiler diagnostics are silent in parser output

[whisperity]

As discovered in #432 with #403 being the potential culprit, the Clang compiler diagnostics during parse time are not printed to the parser's standard streams anymore:

When analysing CodeCompass with CodeCompass itself:

[INFO] (62/129) Parsing /home/w/Work/CodeCompass/Source/webserver/src/session.cpp
[INFO] (63/129) Parsing /home/w/Work/CodeCompass/Source/webserver/src/sessionmanager.cpp
2 warnings generated.
[INFO] (64/129) Parsing /home/w/Work/CodeCompass/Source/webserver/src/threadedmongoose.cpp
[INFO] (65/129) Parsing /home/w/Work/CodeCompass/Source/webserver/src/mongoose.c
[INFO] (66/129) Parsing /home/w/Work/CodeCompass/Source/webserver/authenticators/plain/src/plugin.cpp
2 warnings generated.

[...]

2 errors generated.
Error while processing /home/w/Work/CodeCompass/Source/plugins/cpp/test/src/cppparsertest.cpp.
[WARNING] (100/129) Parsing /home/w/Work/CodeCompass/Source/plugins/cpp/test/src/cppparsertest.cpp has been failed.

Previously, the Clang diagnostics were formatted and output as if a true compiler was running. While it's less of a problem for warnings, for errors it's challenging to understand the output (especially without booting a server) - it says 2 errors generated but never says what the errors are.

[mcserep]

#403 have added DiagnosticConsumer to ClangTool:

clang::tooling::ClangTool tool(*compilationDb, command_.Filename);

DiagnosticMessageHandler diagMsgHandler(_ctx.srcMgr, _ctx.db);
tool.setDiagnosticConsumer(&diagMsgHandler);

In ClangTool the DiagConsumer is nullptr by default and the default operation in that case is to use TextDiagnosticPrinter. This is overridden now.

TextDiagnosticPrinter DiagnosticPrinter(llvm::errs(), &*DiagOpts);
DiagnosticsEngine Diagnostics(IntrusiveRefCntPtr<DiagnosticIDs>(new DiagnosticIDs()), &*DiagOpts,
       DiagConsumer ? DiagConsumer : &DiagnosticPrinter, false);

So our own DiagnosticMessageHandler shall include printing out these messages if we want this feature.

[mcserep]

Maybe inherit DiagnosticMessageHandler from TextDiagnosticPrinter instead of the abstract base class DiagnosticConsumer?