Unify the codepaths used to verify base and member initializers for explicitly

and implicitly defined constructors.  This has a number of benefits:

1. Less code.

2. Explicit and implicit constructors get the same diagnostics.

3. The AST explicitly contains constructor calls from implicit default
constructors.  This allows handing some cases that previously weren't handled
correctly in IRGen without any additional code. Specifically, implicit default
constructors containing calls to constructors with default arguments are now
handled correctly.

llvm-svn: 86500