In the generated CSA, we called overloaded macros while relying on CSA
subtyping of TNodes. This doesn't work well with overloads, because
for C++ any TNode subtyping is treated as an implicit conversion, which
makes these calls ambiguous for C++.
As a solution, we insert implicit_cast conversions for arguments
according to the type predicted by Torque. This way, a CSA overload is always
called with exactly the signature declared in base.tq.
This has the additional benefit that it validates the signatures declared in
base.tq, which could previously be too permissive.
Also, this triggered a bug in structs, where VisitResult's were
carrying the wrong type.

Bug: v8:7793
TBR: danno@chromium.org
Change-Id: I8ed4bfd04793c8a8805a4a3dd5cf2a85c20ce786
Reviewed-on: https://chromium-review.googlesource.com/1165237
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54948}