[Sema][CodeComplete][ObjC] Don't split the first selector fragment

Summary:
Standardize the formatting of selector fragments to include the ':',
e.g. for `- (void)foobar:(int)foobar;`, report `{foobar:}` instead of
`{foobar}{:}`. This was normally the case except for a couple of places
where it was split.

This also improves integration with clangd since it relies upon the `:`
to identify ObjC selectors.

NOTE: It is possible to have selector fragments that are just `:` with
no text, we now handle this properly for the first fragment.

Reviewers: sammccall, doug.gregor

Subscribers: ilya-biryukov, dexonsmith, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D82306

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