devel/llvm16: backport flang fixes for llvm19

The FLANG option is off by default so this wasn't noted in the llvm19
import. Backport 7bc7672925f81 from upstream LLVM.

The patches to fold-real.cpp and reduction-templates.h were hand
applied.

PR: 283966
Reported by: ohartmann@walstatt.org

(cherry picked from commit 9592edb43243a6a2bdef59c99a69e5a309df910f)