Port TemplateStringsArray immutability to release-2.0

[DanielRosenwasser]

Ports #9561 to release-2.0.

[jkillian]

This forces all users to change any template string tag functions, correct?

For example, in TSLint we have:

export function dedent(strings: string[], ...values: string[]) {

which now needs to be changed to

export function dedent(strings: TemplateStringsArray, ...values: string[]) {

[jkillian]

On further examination, this is fine by me 👍
I had initially thought that TemplateStringsArray was a new type introduced for 2.0. But I see that it existed before, and I can (and should) use the same type with 1.8 compatible code.