Implement initialization of a reference (C++ [dcl.init.ref]) as part

of copy initialization. Other pieces of the puzzle:

  - Try/Perform-ImplicitConversion now handles implicit conversions
    that don't involve references.
  - Try/Perform-CopyInitialization uses
    CheckSingleAssignmentConstraints for C. PerformCopyInitialization
    is now used for all argument passing and returning values from a
    function.
  - Diagnose errors with declaring references and const values without
    an initializer. (Uses a new Action callback, ActOnUninitializedDecl).
  
We do not yet have implicit conversion sequences for reference
binding, which means that we don't have any overloading support for
reference parameters yet.

llvm-svn: 58353