[PATCH 4/5] diagnostics: add diagnostic_context::get_location_text

[David Malcolm]

On Tue, 2023-11-21 at 17:20 -0500, David Malcolm wrote:
> No functional change intended.
> 
> gcc/ChangeLog:
>         * diagnostic.cc (diagnostic_get_location_text): Convert to...
>         (diagnostic_context::get_location_text): ...this, and convert
>         return type from char * to label_text.
>         (diagnostic_build_prefix): Update for above change.
>         (default_diagnostic_start_span_fn): Likewise.
>         (selftest::assert_location_text): Likewise.
>         * diagnostic.h (diagnostic_context::get_location_text): New
> decl.

Successfully bootstrapped & regrtested on x86_64-pc-linux-gnu.

Not a bugfix, but low-risk and useful for libdiagnostics, so I've taken
the liberty of pushing this to gcc trunk as r14-5895-g93096d3ce14a89.

Dave