unknown types should not able to concat with strings

TypeScript Version: 3.3.0

Code

declare const a: unknown;
const b = a + 123;
const c = a + '123';
const d = `${a}123`;

Expected behavior:
All the declarations of variables b c d should have errors.

Actual behavior:
Only declaration of variable b have error.

Playground Link:
https://www.typescriptlang.org/play/index.html#src=declare%20const%20a%3A%20unknown%3B%0Aconst%20b%20%3D%20a%20%2B%20123%3B%0Aconst%20c%20%3D%20a%20%2B%20'123'%3B%0Aconst%20d%20%3D%20%60%24%7Ba%7D123%60%3B%0A

Although we won't get runtime errors in this case, it may cause unpredictable result, e.g. "[[Object]]123", which is unexpected, so I think ts should not allow unknown types to concat with strings.

Start with the TypeScript Playground reproduction and compare the diagnostics for b, c, and d when a is typed as unknown. Trace the type-checking behavior for arithmetic, string concatenation, and template expressions. Done means the behavior matches the stated expected errors and the relevant checks are covered.