[test][asan] Make the printf-5.c test case actually emit a volatile memcpy.

The current test in printf-5.c appears to try to emit a volatile memcpy for the
format string, but it doesn't because the volatile qualifier is implicitly
casted away. Using a string literal instead preserves the volatile qualifier.

This is a follow-up to D137031 and is a prerequisite for D136822, which elides
memcpys in more instances and would otherwise break this test.

Differential Revision: https://reviews.llvm.org/D137042