Avoid creating ConstantComposites with spec constants (#3662)
A recent version of spirv-val is now reporting "OpConstantComposite must
not have spec constant operands" for some tests.
Fix by letting `addCompositeConstant` defer to
`addSpecConstantComposite` when attempting to add a constant that has
one or more specialization constant operands.
Original commit:
https://github.com/KhronosGroup/SPIRV-LLVM-Translator/commit/13f1c79e6272d49
