Enforce simple reactive function call rules

Enforce simple reactive function call rules

Summary:
Enforce the reactivity rules for function calls between rx, rx-shallow,
rx-local and non-rx functions. RxEnforceCalls option can be used to control the
enforcement (0 = off, 1 = warning, 2 = exception).

Limitations:

Conditional reactivity is not tracked yet, so we pretend every conditionally
reactive caller is non-reactive and every conditionally reactive callee is as
reactive as possible. This is needed to prevent spurious warnings/exceptions
before conditional reactivity is implemented.

Calls that reenter VM are another known source of enforcement holes not
addressed by this diff.

Usage of Rx\IS_ENABLED is not tracked yet either and this is a known source
of spurious warnings.

Rx-incompatible callee may get folded by hhbbc, skipping rx check. The
next diff fixes this.

Reviewed By: markw65

Differential Revision: D10443336

fbshipit-source-id: 869e1c796ab6b6be7cdfeae979e80c6f91852ec4