devel/llvm15: 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: 285195
Reported by: milios@ccsys.com

(cherry picked from commit ce2f720d96cde31a9d6c3801a1b3dc583e1d23e3)