This changes the contract of the CallFunctionTemplate builtins such that
the receiver must always be a JSReceiver, which simplifies the logic and
also makes it possible to have TurboFan completely optimize away the
ConvertReceiver operation, while at the same time ensuring that we
always pass the correct receiver to API functions.

Bug: chromium:961199, v8:8820
Change-Id: Ied9dee3f504bd5b711c29f1e2c00f0434c731d19
Cq-Include-Trybots: luci.chromium.try:linux-rel,win7-rel
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1611803
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61563}