gcc conventionally does not upper case in error messages.

char_lit1.go:13: error: invalid unicode code point 0xd800
char_lit1.go:14: error: invalid unicode code point 0xd999
char_lit1.go:15: error: invalid unicode code point 0xdc01
char_lit1.go:16: error: invalid unicode code point 0xdddd
char_lit1.go:17: error: invalid unicode code point 0xdfff
char_lit1.go:20: error: invalid unicode code point 0x110000
char_lit1.go:22: error: invalid unicode code point 0x110000
char_lit1.go:23: error: invalid unicode code point 0xffffffff

R=rsc
CC=golang-dev
https://golang.org/cl/2040042