compiler: fix buglet in function inlining related to sink names

compiler: fix buglet in function inlining related to sink names

    When the compiler writes an inlinable function to the export data,
    parameter names are written out (in Export::write_name) using the
    Gogo::message_name as opposed to a raw/encoded name. This means that
    sink parameters (those named "_") get created with the name "_"
    instead of "._" (the name created by the lexer/parser). This confuses
    Gogo::is_sink_name, which looks for the latter sequence and not just
    "_". This can cause issues later on if an inlinable function is
    imported and fed through the rest of the compiler (things that are
    sinks are no recognized as such). To fix these issues, change
    Gogo::is_sink_name to return true for either variants ("_" or "._").

    Fixes golang/go#35586.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/207259

From-SVN: r278275