Objective-C properties: merge attributes when redeclaring 'readonly' as 'readwrite...

Objective-C properties: merge attributes when redeclaring 'readonly' as 'readwrite' in an extension.

r251874 stopped back-patching the AST when an Objective-C 'readonly'
property is redeclared in a class extension as 'readwrite'. However,
it did not properly handle merging of Objective-C property attributes
(e.g., getter name, ownership, atomicity) to the redeclaration,
leading to bad metadata. Merge (and check!) those property attributes
so we get the right metadata and reasonable ASTs. Fixes
rdar://problem/23823989.

llvm-svn: 255309