[DebugInfo] Add kind of ImplicitParamDecl for emission of FlagObjectPointer.

Summary:
If the first parameter of the function is the ImplicitParamDecl, codegen
automatically marks it as an implicit argument with `this` or `self`
pointer. Added internal kind of the ImplicitParamDecl to separate
'this', 'self', 'vtt' and other implicit parameters from other kind of
parameters.

Reviewers: rjmccall, aaron.ballman

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D33735

llvm-svn: 305075