Initial support for formatting trailing return types.

This fixes llvm.org/PR15170.

For now, the basic formatting rules are (based on the C++11 standard):
* Surround the "->" with spaces.
* Break before "->".

Also fix typo.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@185938 91177308-0d34-0410-b5e6-96231b3b80d8