compiler: Disallow rune literals with '\X'.

    
    http://golang.org/ref/spec#Rune_literals states	that rune literals
    encoded as hex bytes must begin with lowercase '\x'.
    Fixes golang/go#11575.
    
    Reviewed-on: https://go-review.googlesource.com/12158

From-SVN: r226184

gcc/go/gofrontend/MERGE

-ac462880e803a926005f1756b0f8d82ff0c47499
+46117382a58843af60fc2feab68c433a96f79e79
 
 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

@@ -1197,7 +1197,6 @@ Lex::advance_one_char(const char* p, bool is_single_quote, unsigned int* value,
 		  : p + 1);
 
 	case 'x':
-	case 'X':
 	  *is_character = false;
 	  if (Lex::is_hex_digit(p[1]) && Lex::is_hex_digit(p[2]))
 	    {