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

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-1e2d98b27701744cf0ec57b19d7fc8f594184b9a
+2d0504236c7236345ee17a0cb43a3bb9ce3acf7f
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/gogo.h

@@ -222,7 +222,9 @@ class Gogo
   {
     return (name[0] == '.'
 	    && name[name.length() - 1] == '_'
-	    && name[name.length() - 2] == '.');
+	    && name[name.length() - 2] == '.')
+        || (name[0] == '_'
+            && name.length() == 1);
   }
 
   // Helper used when adding parameters (including receiver param) to the