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
