[FormatManager] Move Language lookup into the obviously non-cached part (NFC)

This refactoring makes the lookup caching easier to reason about. This
has no observable effect although it does slightly change what is
being cached.

- Before this patch a negative lookup in the LanguageCategory would be
  cached, but a positive wouldn't.

- After this patch LanguageCategory lookups aren't cached by
  FormatManager, period. (LanguageCategory has its own FormatCache for this!)

Differential Revision: https://reviews.llvm.org/D71289