[CMake] Disable -Wdangling-reference warnings on GCC (#157541)

This gets rid of 99 warnings which mostly seem like false positives (in
a build of LLVM+Clang+LLDB, with GCC 13).

The warnings look e.g. like this:

../lib/ObjCopy/COFF/COFFObjcopy.cpp: In function ‘uint64_t
llvm::objcopy::coff::getNextRVA(const Object&)’:
../lib/ObjCopy/COFF/COFFObjcopy.cpp:38:18: warning: possibly dangling
reference to a temporary [-Wdangling-reference]
       38 |   const Section &Last = Obj.getSections().back();
          |                  ^~~~
../lib/ObjCopy/COFF/COFFObjcopy.cpp:38:47: note: the temporary was
destroyed at the end of the full expression ‘(&
Obj)->llvm::objcopy::coff::Object::getSections().llvm::ArrayRef<llvm::objcopy::coff::Section>::back()’
       38 |   const Section &Last = Obj.getSections().back();
          |                         ~~~~~~~~~~~~~~~~~~~~~~^~

In this example, the `Object::getSections()` method returns an
`ArrayRef<Section>` from a `std::vector<Section>`. We invoke `back()` on
that, and store a reference in a local variable. Even though the
temporary `ArrayRef<Section>` has been destroyed, the reference points
to something which still is alive in the `std::vector<Section>`.

See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109642 and
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110358 for some
preexisting discussion on this warning and how it can be silenced
selectively since GCC 14.