MSVC: enable identical COMDAT folding for release builds

According to Microsoft's docs, /OPT:ICF is enabled by the
linker by default unless /OPT:NOICF or /DEBUG is specified.

If we are in RelWithDebInfo mode, /DEBUG is passed to the
linker to generate debug symbols, however, it caused the
identical COMDAT folding be disabled. We now pass /OPT:ICF
to the linker explicitly to prevent this.

[1] https://docs.microsoft.com/en-us/cpp/build/reference/opt-optimizations?view=msvc-160

Change-Id: I02099edb81034ace7bb19f1164d57829e3979a5f
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

cmake/QtInternalTargets.cmake

@@ -194,7 +194,7 @@ if (MSVC)
 
     target_link_options(PlatformCommonInternal INTERFACE
         -DYNAMICBASE -NXCOMPAT
-        $<$<NOT:$<CONFIG:Debug>>:-OPT:REF>
+        $<$<NOT:$<CONFIG:Debug>>:-OPT:REF -OPT:ICF>
     )
 endif()
 

mkspecs/common/msvc-desktop.conf

@@ -85,8 +85,8 @@ QMAKE_RUN_CXX_IMP_BATCH = $(CXX) -c $(CXXFLAGS) $(INCPATH) -Fo$@ @<<
 
 QMAKE_LINK              = link
 QMAKE_LFLAGS            = /NOLOGO /DYNAMICBASE /NXCOMPAT
-QMAKE_LFLAGS_RELEASE    = /OPT:REF /INCREMENTAL:NO
-QMAKE_LFLAGS_RELEASE_WITH_DEBUGINFO = /DEBUG /OPT:REF /INCREMENTAL:NO
+QMAKE_LFLAGS_RELEASE    = /OPT:REF /OPT:ICF /INCREMENTAL:NO
+QMAKE_LFLAGS_RELEASE_WITH_DEBUGINFO = $$QMAKE_LFLAGS_RELEASE /DEBUG
 QMAKE_LFLAGS_DEBUG      = /DEBUG
 QMAKE_LFLAGS_CONSOLE    = /SUBSYSTEM:CONSOLE
 QMAKE_LFLAGS_WINDOWS    = /SUBSYSTEM:WINDOWS