CMake build: Force optimization of CPlusPlus lib

Even in the debug build, as done for the qmake build system. This is for
performance optimization of this critical part for C++ parsing even in
debug / developer builds.

Change-Id: I9552ba9fc44e213f2df1d2d2a64a126af3603fd1
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: Christian Stenger <christian.stenger@qt.io>
Reviewed-by: Cristian Adam <cristian.adam@qt.io>

src/libs/cplusplus/CMakeLists.txt

@@ -40,4 +40,13 @@ add_qtc_library(CPlusPlus
     pp-engine.h pp-scanner.cpp
     pp-scanner.h
     pp.h
+  SKIP_PCH
 )
+
+if(TARGET CPlusPlus)
+  qtc_enable_release_for_debug_configuration()
+  if (BUILD_WITH_PCH)
+    target_precompile_headers(CPlusPlus PRIVATE
+      "${QtCreator_SOURCE_DIR}/src/shared/qtcreator_gui_pch.h")
+  endif()
+endif()