SL-16094: Statically link to Windows IMM32.LIB.

llwindowwin32.cpp's LLWinImm class used to dynamically load IMM32.DLL and
populate its methods using GetProcAddress(). That was to support Windows XP.
Since we've dropped Windows XP, use static linking instead, with dramatically
fewer lines of code (and less of a thread safety alarm trigger).

We retain the LLWinImm wrapper class only as a hook for Tracy instrumentation.