clang-format: keep ObjC colon alignment with short object name

Summary:
When the target object expression is short and the first selector name
is long, clang-format used to break the colon alignment:

  [I performSelectorOnMainThread:@selector(loadAccessories)
                       withObject:nil
                    waitUntilDone:false];

This happens because the colon is placed at `ContinuationIndent +
LongestObjCSelectorName`, so that any selector can be wrapped. This is
however not needed in case the longest selector is the firstone, and
not wrapped.

To overcome this, this patch does not include the first selector in
`LongestObjCSelectorName` computation (in TokenAnnotator), and lets
`ContinuationIndenter` decide how to account for the first selector
when wrapping. (Note this was already partly the case, see line 521
of ContinuationIndenter.cpp)

This way, the code gets properly aligned whenever possible without
breaking the continuation indent.

  [I performSelectorOnMainThread:@selector(loadAccessories)
                      withObject:nil
                   waitUntilDone:false];
  [I // force break
      performSelectorOnMainThread:@selector(loadAccessories)
                       withObject:nil
                    waitUntilDone:false];
  [I perform:@selector(loadAccessories)
      withSelectorOnMainThread:true
                 waitUntilDone:false];

Reviewers: krasimir, djasper, klimek

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D43121

llvm-svn: 324741