jit: always native_call {Class|Trait|Interface}Exists helper

Summary: ... instead of punting to the interpreter when we can't
 determine type information. The builtin version we end up calling
 (string) and (bool) casts the name and autoload params, which
 bytecode can do just fine.

Reviewed By: @jdelong

Differential Revision: D1337358
make {Class|Trait|Interface}Exists a single bytecode

Summary: ... it might've made sense to make three IR opcodes for these
 three cases, but having three bytecodes that are exactly the same
 is excessive.

Reviewed By: @jdelong

Differential Revision: D1343606
Cleaned HipHop prefix in error messages

Summary: I don't want to bust out a rap when I encounter an error ;-)

Also changed `Recoverable Fatal error` to `Catchable Fatal error` so it matches php
Closes #2747

Reviewed By: @fredemmott

Differential Revision: D1346698

Pulled By: @JoelMarcey
Only define SID when needed

Summary: SID was defined regardless if there was a session or not. PHP only defines it if needed, so should we

Fixes #1915
Fixes #2705

Reviewed By: @fredemmott

Differential Revision: D1337456