Reintroduce precompiled headers

[xokdvium]

Motivation

A lot of compile time can be shaved off by just using using precompiled headers for expensive files.
Bottlenecks were discovered with the help of #13510.

Context

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[xokdvium]

On my machine this shaves off quite a few CPU minutes for native-clangStdenv (though the wall time isn't affected much, but it would be for less powerful machines):

(Before)

Command being timed: "ninja"
        User time (seconds): 2122.56
        System time (seconds): 111.07
        Percent of CPU this job got: 1278%
        Elapsed (wall clock) time (h:mm:ss or m:ss): 2:54.70

(After)

Command being timed: "ninja"
        User time (seconds): 1695.70
        System time (seconds): 89.14
        Percent of CPU this job got: 1058%
        Elapsed (wall clock) time (h:mm:ss or m:ss): 2:48.66

[roberth]

Amazing

[Ericson2314]

Thank you!