"follow symbol under cursor" does not work when unity builds are enabled.

steps to reproduce:

• configure qtbase with unity builds enabled
• in qstring.h, find the QString::truncate declaration, "follow symbol under cursor" does nothing

it does work for c++ files that are excluded from unity builds: e.g. opening qpicture.h, "follow symbol under cursor" on QPicture::load will switch to qpicture.cpp (this is on linux where qpicture.cpp is excluded from unity builds)