[AST] Unify the code paths of traversing lambda expressions.

Summary:
This supposes to be a non-functional change. We have two code paths when
traversing lambda expressions:

1) traverse the function proto typeloc when parameters and return type
are explicit;
2) otherwise fallback to traverse parameter decls and return type loc
individually;

This patch unifies the code path to always traverse parameters and
return type, rather than relying on traversing the full type-loc.

Reviewers: ilya-biryukov

Subscribers: arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D55820

llvm-svn: 349494