build: remove approximately 400 seconds of CPU time (30%)

This took parsing time from 1421s or so to 1060s or so. The reason is
entirely nlohmann. All of the stuff below is just Obliterated because it's
built in the PCH instead:

**** Templates that took longest to instantiate:
219051 ms: nlohmann::basic_json<>::parse<const char *> (276 times, avg 793 ms)
169675 ms: nlohmann::basic_json<>::basic_json (1127 times, avg 150 ms)
129416 ms: nlohmann::detail::parser<nlohmann::basic_json<>, nlohmann::detail::i... (276 times, avg 468 ms)
 98155 ms: nlohmann::detail::parser<nlohmann::basic_json<>, nlohmann::detail::i... (276 times, avg 355 ms)
 81322 ms: nlohmann::basic_json<>::json_value::json_value (1405 times, avg 57 ms)
 53531 ms: nlohmann::detail::json_sax_dom_callback_parser<nlohmann::basic_json<... (276 times, avg 193 ms)

clang-only. This brings the clang build time to not far from *half* of
the gcc build time.

Also, clang does not enjoy so much to miscompile coroutines. Maybe we
should just be clang-only.

Change-Id: Id3135db0094e4560830674090e32e6da2c22fcc6

meson.build

@@ -147,6 +147,17 @@ if should_pch
   # Unlike basically everything else that takes a file, Meson requires the arguments to
   # cpp_pch : to be strings and doesn't accept files(). So absolute path it is.
   cpp_pch = [meson.project_source_root() / 'src/pch/precompiled-headers.hh']
+
+  # Saves about 400s (30% at time of writing) from compile time on-cpu, mostly
+  # by removing instantiations of nlohmann from every single damned compilation
+  # unit.
+  # There is no equivalent in gcc.
+  if cxx.get_id() == 'clang'
+    add_project_arguments(
+      '-fpch-instantiate-templates',
+      language : 'cpp',
+    )
+  endif
 else
   cpp_pch = []
 endif