build-time: remove 20% more by PCH'ing C++ stdlib

It seems like someone implemented precompiled headers a long time ago
and then it never got ported to meson or maybe didn't work at all.

This is, however, blessedly easy to simply implement. I went looking for
`#define` that could affect the result of precompiling the headers, and
as far as I can tell we aren't doing any of that, so this should truly
just be free build time savings.

Previous state:
Compilation (551 times):
  Parsing (frontend):         1302.1 s
  Codegen & opts (backend):    956.3 s

New state:
**** Time summary:
Compilation (567 times):
  Parsing (frontend):         1123.0 s
  Codegen & opts (backend):   1078.1 s

I wonder if the "regression" in codegen time is just doing the PCH
operation a few times, because meson does it per-target.

Change-Id: I664366b8069bab4851308b3a7571bea97ac64022

0 files changed, 0 insertions, 0 deletions