meson: Further optimize compile times with PCH template instantiations

[xokdvium]

Motivation

This is a follow-up to 6ec50ba, which also almost halves the compile times on clang for subprojects that use PCH.

-fpch-instantiate-templates is a clang-only option to force the instantiation of templates once in the PCH itself, not all of the translation units that it gets included to. This really cuts down on the overhead from nlohmann::json and std::format code:

 48244 ms: nlohmann::basic_json<>::parse<const char *> (76 times, avg 634 ms)
 36193 ms: nlohmann::basic_json<>::basic_json (310 times, avg 116 ms)
 28307 ms: nlohmann::detail::parser<nlohmann::basic_json<>, nlohmann::detail::i... (76 times, avg 372 ms)
 20334 ms: nlohmann::detail::parser<nlohmann::basic_json<>, nlohmann::detail::i... (76 times, avg 267 ms)
 17387 ms: nlohmann::basic_json<>::json_value::json_value (389 times, avg 44 ms)
 16822 ms: std::vformat_to<std::__format::_Sink_iter<char>> (76 times, avg 221 ms)
 16771 ms: std::__format::__do_vformat_to<std::__format::_Sink_iter<char>, char... (76 times, avg 220 ms)
 12160 ms: std::vformat_to<std::__format::_Sink_iter<wchar_t>> (76 times, avg 160 ms)
 12127 ms: std::__format::__do_vformat_to<std::__format::_Sink_iter<wchar_t>, w... (76 times, avg 159 ms)
 10397 ms: nlohmann::detail::json_sax_dom_callback_parser<nlohmann::basic_json<... (76 times, avg 136 ms)
  9118 ms: nlohmann::basic_json<>::data::data (76 times, avg 119 ms)

Initially done by Jade Lovelace lix@jade.fyi in https://gerrit.lix.systems/c/lix/+/1842. We are doing basically the same, but unconditionally. It would be a huge pain to add a pch option for all subprojects to just support the usecase of using clangd in a gcc devshell.

In total, this basically halves the frontend times for nix-util and nix-store to the point that the most expensive part of the build is linking.

(nix-store before):

**** Time summary:
Compilation (77 times):
  Parsing (frontend):          243.4 s
  Codegen & opts (backend):    140.3 s

(nix-store after):

**** Time summary:
Compilation (77 times):
  Parsing (frontend):          120.2 s
  Codegen & opts (backend):    141.2 s

Context

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