[Clang][NeonEmitter] emit ret decl first for -Wdeclaration-after-statement

The generated arm_neon.h header isn't -Wdeclaration-after-statement
compliant when targeting -mbig-endian. Update the generator to declare
the return value, if any, first before any other arguments that might
need to be "reversed" from little endian to big.

Another approach would have been to try to ignore this warning in system
headers, though that might not be precise for tokens involved in macro
expansion. See also: https://reviews.llvm.org/D116833#3236209.

Link: https://github.com/ClangBuiltLinux/linux/issues/1603
Fixes: https://github.com/llvm/llvm-project/issues/54062

Reviewed By: DavidSpickett

Differential Revision: https://reviews.llvm.org/D122189