[TrailingObjects] Convert ASTTemplateKWAndArgsInfo and ASTTemplateArgumentListInfo.

Doing so required separating them so that the former doesn't inherit
from the latter anymore. Investigating that, it became clear that the
inheritance wasn't actually providing real value in any case.

So also:
- Remove a bunch of redundant functions (getExplicitTemplateArgs,
  getOptionalExplicitTemplateArgs) on various Expr subclasses which
  depended on the inheritance relationship.
- Switched external callers to use pre-existing accessors that return the
  data they're actually interested in (getTemplateArgs,
  getNumTemplateArgs, etc).
- Switched internal callers to use pre-existing getTemplateKWAndArgsInfo.

llvm-svn: 256359