ClangCodeModel: Use dedicated TU for code related to the clangd AST

The clangdclient.cpp file is rather large, and the AST stuff is a nicely
self-contained chunk of code to move out.
No functional changes.

Change-Id: I0240413f561f5c67ca5ee310b5c4253ffa62fdae
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/clangcodemodel/clangcodemodel.qbs

@@ -30,6 +30,8 @@ QtcPlugin {
         "clangcompletioncontextanalyzer.cpp",
         "clangcompletioncontextanalyzer.h",
         "clangconstants.h",
+        "clangdast.cpp",
+        "clangdast.h",
         "clangdclient.cpp",
         "clangdclient.h",
         "clangdiagnostictooltipwidget.cpp",