compiler: better error for unexpected digit

A digit character is not invalid in general, but it's not permitted
at the start of an identifier.  Report a better error message.
The test case is issue11359.go in the source repo.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/278174

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-3e8f49a2137a87fdaba51c3002ddbe41ab18ed46
+8b913a1865e36d4bd105f29aa0b12264a4e03515
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/lex.cc

@@ -743,6 +743,13 @@ Lex::next_token()
 		if (Lex::is_unicode_letter(ci))
 		  return this->gather_identifier();
 
+		if (!issued_error && Lex::is_unicode_digit(ci))
+		  {
+		    go_error_at(this->location(),
+				"identifier cannot begin with digit");
+		    issued_error = true;
+		  }
+
 		if (!issued_error)
 		  go_error_at(this->location(),
 			      "invalid character 0x%x in input file",