Remove string literals from unions with matching template literals

[Anders Hejlsberg (ahejlsberg)]

We currently remove template literals from intersections containing matching string literals. For example `foo${string}` & 'foox' is reduced to just 'foox'. We also need to perform the complementary reduction for union types, i.e. `foo${string}` | 'foox' should reduce to just `foo${string}`. This PR adds that functionality.

The following now produces no errors:

var x: '0';
var x: '0' & `${number}`;

var y: `${number}`;
var y: `${number}` | '0';  // Ok, previously was error

See also #40598.