cases: fix case mappings to allow for Gregorian uppercase

Prepare for Unicode 11, where in some cases
an undefined title case is lower case, instead of
upper case.

Updates golang/go#27945

Change-Id: I96d6921434e344271c80dbc4033b5952b8284178
Reviewed-on: https://go-review.googlesource.com/c/text/+/169640
Run-TryBot: Marcel van Lohuizen <[email protected]>
Reviewed-by: Russ Cox <[email protected]>

Author: mpvl

cases/gen.go

@@ -355,7 +355,7 @@ func makeException(ri *runeInfo) {
 			log.Fatalf("%U: has zero-length mapping.", ri.Rune)
 		}
 		*b <<= 3
-		if s != orig {
+		if s != orig || ri.CaseMode == cLower {
 			n := len(s)
 			if n > 7 {
 				log.Fatalf("%U: mapping larger than 7 (%d)", ri.Rune, n)
@@ -384,12 +384,6 @@ func makeException(ri *runeInfo) {
 		addString(uc, &exceptionData[p])
 	}
 	if ct != cTitle {
-		// If title is the same as upper, we set it to the original string so
-		// that it will be marked as not present. This implies title case is
-		// the same as upper case.
-		if tc == uc {
-			tc = orig
-		}
 		addString(tc, &exceptionData[p])
 	}
 }