PCH: Document and test COMPILE_LANGUAGE genex for per-language header

Fixes: #19839
Status Job ID Name Coverage
  External
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passed cmake-dash3win10-windows-release-ninja

00:18:36

passed cmake-hythloth-linux-debug-makefiles+sphinx

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passed cmake-minoc-linux-release-lint-iwyu-tidy

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passed cmake-moonshade-osx-debug-ninja

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passed cmake-moonshade-osx-release-makefiles

00:11:02

passed cmake-terfin-windows-release-vs2019

00:35:19

passed ghostflow-check-master

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