[es6] Introduce spec compliant IsConstructor.

[es6] Introduce spec compliant IsConstructor.

There was already a bit on the Map named "function with prototype",
which basically meant that the Map was a map for a JSFunction that could
be used as a constructor. Now this CL generalizes that bit to
IsConstructor, which says that whatever (Heap)Object you are looking at
can be used as a constructor (i.e. the bit is also set for bound
functions that can be used as constructors and proxies that have a
[[Construct]] internal method).

This way we have a single chokepoint for IsConstructor checking, which
allows us to get rid of the various ways in which we tried to guess
whether something could be used as a constructor or not.

Drive-by-fix: Renamed IsConstructor on FunctionKind to
IsClassConstructor to resolve the weird name clash, and the
IsClassConstructor name also matches the spec.

R=jarin@chromium.org, rossberg@chromium.org
BUG=v8:4430
LOG=n

Review URL: https://codereview.chromium.org/1358423002

Cr-Commit-Position: refs/heads/master@{#30900}