Rename Selector::getName() to Selector::getAsString(), and add

Rename Selector::getName() to Selector::getAsString(), and add
a new NamedDecl::getAsString() method.

Change uses of Selector::getName() to just pass in a Selector
where possible (e.g. to diagnostics) instead of going through
an std::string.

This also adds new formatters for objcinstance and objcclass
as described in the dox.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@59933 91177308-0d34-0410-b5e6-96231b3b80d8