swift234

Add "single extended grapheme cluster" literals (SEGCL) -- a subset of
double-quoted string literals that contain a single extended grapheme cluster

SEGCL by default infer type String, but you can ask to infer Character
for them.

Single quoted literals continue to infer Character.

Actual extended grapheme cluster segmentation is not implemented yet,
<rdar://problem/16755123> Implement extended grapheme cluster
segmentation in libSwiftBasic

This is part of
<rdar://problem/16363872> Remove single quoted characters

Swift SVN r17034