Adds 'symbol constraints' for easier checking of assignments.

[jbondc]

I'll work on a proposal to explain what this solves:

SymbolConstraints

• Adds extra bits since none left in SymbolFlags
• Gives an easy way to check assignments to something that is notWritable

Fixes #2215
Solves: #2456, #12, ?

ConstValue

• Gives some knowledge that a Node has a constant value.

[jbondc]

Just realized that enums don't allow assignments:

Assignments to property access in enums is disallowed but indexed access is ok:

Is that intentional? I'd expect the enum to be writable since you can do that at runtime.

The const enum doesn't exist so notWritable.

[mhegazy]

As the comment mentioned, the 1.0 compiler did not do that, we did not want to introduce a breaking change. we also left indexed access to be a "escape hatch" from the type checking per se.

[mhegazy]

I do not think this is the correct approach. the check should be done in "checkReferenceExpression" in "isReferenceOrErrorExpression" checking that a const enum element access expression is not a valid left hand side expression

[mhegazy]

i will close this one, please resubmit with changes to checkReferenceExpression.