[Bug c++/89326] [RFE] Highlight `required from here` in compile-error output

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89326

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I think it highlights all lines where the location is the main source file, so
they stand out from anything with a location in a header.