Optional chaining

[ShortDevelopment]

This PR aims to add support for the stage-4 proposal optional-chaining.
It's inspired by the work of @rhuanjl but uses a more hacky approach to parsing.

Goals

• Minimize amount of changes
• (Hopefully) Performance

ToDo

• Add tests
  • Unused expression result
  • Return expression
  • Root optional-call (e.g. eval?.('a'))
  • Scoped method load (e.g. inside eval)
  • delete
• implement optional-deletion
• optional call invoked on eval function should be indirect eval
• Simple function calls
• Preserve this
(a.b)().c should be equivalent to a.b().c
• short circuit embedded expressions like a?.[++x]
++x should not be evaluated if a is null or undefined
• Don't tokenize a?.3:0 (ternary) as tkOptChain (?.)
• Tagged templates are not allowed in optional chain
• fix tmp-renaming for eval result
  eval("foo?.()") or eval("foo?.property")
• Works with optimizations
  • What about the apply call optimization?
  • Loop inversion (AST cloning)
    Only triggered for 2 nested for loops with assignment
    

    ChakraCore/test/loop/loopinversion.js

    Lines 60 to 66 in e26c81f

    	for (var j = 0; j < 1; ++j) {
    	for (var i = 0; i < 1; ++i) {
    	1;
    	c = 2;
    	1;
    	}
    	};

Out of scope

• Remove EmitInvoke (seems unused)
• Better error message for a call on a non-function

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Spec: Optional Chains
Fixes #6349