Eliminate all of the placeholder identifiers used for constructors,

Eliminate all of the placeholder identifiers used for constructors,
destructors, and conversion functions. The placeholders were used to
work around the fact that the parser and some of Sema really wanted
declarators to have simple identifiers; now, the code that deals with
declarators will use DeclarationNames.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@59469 91177308-0d34-0410-b5e6-96231b3b80d8