[clang] Fix -Wunused-variable in CGCall.cpp (NFC)

llvm-project/clang/lib/CodeGen/CGCall.cpp:3226:24:
error: unused variable 'StructSize' [-Werror,-Wunused-variable]
        llvm::TypeSize StructSize = CGM.getDataLayout().getTypeAllocSize(STy);
                       ^
llvm-project/clang/lib/CodeGen/CGCall.cpp:3227:24:
error: unused variable 'PtrElementSize' [-Werror,-Wunused-variable]
        llvm::TypeSize PtrElementSize =
                       ^
llvm-project/clang/lib/CodeGen/CGCall.cpp:5313:24:
error: unused variable 'SrcTypeSize' [-Werror,-Wunused-variable]
        llvm::TypeSize SrcTypeSize =
                       ^
llvm-project/clang/lib/CodeGen/CGCall.cpp:5315:24:
error: unused variable 'DstTypeSize' [-Werror,-Wunused-variable]
        llvm::TypeSize DstTypeSize = CGM.getDataLayout().getTypeAllocSize(STy);
                       ^
4 errors generated.