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

misc/pre-commit.nix

@@ -87,11 +87,12 @@ pre-commit-run {
         "file"
         "header"
       ];
-      # generated files; these will never actually be seen by this
-      # check, and are left here as documentation
       excludes = [
-        "(parser|lexer)-tab\\.hh$"
-        "\\.gen\\.hh$"
+        ''^src/pch/.*$''
+        # generated files; these will never actually be seen by this
+        # check, and are left here as documentation
+        ''(parser|lexer)-tab\.hh$''
+        ''\.gen\.hh$''
       ];
       entry = lib.getExe pkgs.check-headers;
     };