[FLANG][NFC]Use RTNAME instead of hard-coding for simplify intrinsics

Use the RTNMAE macro (via stringify macros) to generate the name
strings for runtime functions, instead of using strings.
The sequence of macros generate exactly the same string as the
ones used previously, but this will support future changes in
runtime function names.

No functional change.

Reviewed By: vzakhari

Differential Revision: https://reviews.llvm.org/D132652