Clang optimiser vs preproc.c

[Thomas Munro]

Hi,

FYI, it looks like there is a big jump in CPU time to compile preproc.c at -O2:

clang15: ~16s
clang16: ~211s
clang17: ~233s

First noticed on FreeBSD (where the system cc is now clang16), but
reproduced also on Debian (via apt.llvm.org packages).