Improve code-completion results for the flags in an @property

Improve code-completion results for the flags in an @property
declaration by providing patterns for "getter = <method>" and "setter
= <method>". As part of this, invented a new "pattern" result kind
that is merely a semantic string. The "pattern" result kind should
help with other kinds of code templates.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@89277 91177308-0d34-0410-b5e6-96231b3b80d8