swift234

[ClangImporter] Rewrite enum prefix stripping to use the camel_case helpers.

Previously, we didn't correctly handle acronyms as single words, leading to
importing the NS16BitLittleEndianBitmapFormat enumerator of NSBitmapFormat as
"S16BitLittleEndianBitmapFormat". Now we get "NS16BitLittleEndianBitmapFormat",
which is a little better. See the examples for a few more ways this shakes out.

<rdar://problem/16683848>

Swift SVN r16682