Removed unused variable [NFC]

The compiler warned:
../tools/clang/lib/Sema/SemaType.cpp:6788:31: error: unused variable 'AT' [-Werror,-Wunused-variable]
    if (const AttributedType *AT = S.getCallingConvAttributedType(type)) {
                              ^
1 error generated.

llvm-svn: 340251