Fix a major inconsistency in the representation of Objective-C

Fix a major inconsistency in the representation of Objective-C
classes, categories, protocols, and class extensions, where the
methods and properties of these entities would be inserted into the
DeclContext in an ordering that doesn't necessarily reflect source
order. The culprits were Sema::ActOnMethodDeclaration(), which did not
perform the insertion of the just-created method declaration into
the DeclContext for these Objective-C entities, and
Sema::ActOnAtEnd(), which inserted all method declarations at the
*end* of the DeclContext.

With this fix in hand, clean up the code-completion actions for
property setters/getters that worked around this brokenness in the AST.

Fixes <rdar://problem/8062781>, where this problem manifested as poor
token-annotation information, but this would have struck again in many
other places.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@122347 91177308-0d34-0410-b5e6-96231b3b80d8