Disallow primitive assignability to indexer of type any

[sandersn]

Fixes #4588.

string/numberIndexTypesRelatedTo needs to prevent primitives from being
assignable to an indexer of type 'any'. However, these two functions take
an apparent type, which no longer has the primitive flag set. I thought of
three ways to provide this information:

1. Pass the original type into string/numberIndexTypesRelatedTo and
   check its flag.
2. Record a boolean isPrimitive before converting to the apparent type,
   and pass it to string/numberIndexTypesRelatedTo.
3. Create a helper function isPrimitive that takes the apparent type and
   compares it to globalString/Number/Boolean/ESSymbolType.

I decided on (1) because it seems like the simplest and safest. But none
of the options are elegant. Please suggest improvements.

[RyanCavanaugh]

Is there a bug tracking this change? I'd like to have more context on what we're saying is desirable/non-desirable

[sandersn]

Yes, I'll remember someday to add a link to the bug number: #4588. And it's an improvement to #4074 which was a little too permissive.

[RyanCavanaugh]

FWIW I was interested in the status of this code

interface X {
    [n: number]: any;
}

var x: X;
x = 'hello';

which seems to still compile under this branch, which is what I'd expect.

[sandersn]

The test I added in assignmentCompat1.ts has this case, but I didn't think about it enough and expected it to fail (I forgot that strings do support numeric indexing in typescript). I'll add another test case like

var x: { [n: number]: any } = false

which should (and does) fail.

[sandersn]

Ping @RyanCavanaugh -- can you take another look now that I added more test cases?

[RyanCavanaugh]

👍