Use NonNullable<T> in more scenarios

[Anders Hejlsberg (ahejlsberg)]

This PR builds on #49119 to use NonNullable<T> in more scenarios in --strictNullChecks mode. The PR also cleans up some compiler internals.

• Instantiations of NonNullable<T> are now only created only if T can possibly be null or undefined. Previously, the compiler would sometimes create NonNullable<T> instantiations even for a T constrained to a non-nullable type.
• Given expressions a and b of type A and B, the expression a || b now has type NonNullable<A> | B if A can possibly be null or undefined.
• Given an expression obj of a generic type T constrained to a nullable type, when an expression obj?.x is used in a context that proves x is not undefined, the type of obj is narrowed to NonNullable<T>.
• Given an expression x of type unknown, the expression x! has type {}.
• The getFalsyFlags function is gone and its functionality folded into getTypeFacts.

Some examples (in --strictNullChecks mode):

functionf1<T>(x: T){lety=x||"hello";// Now NonNullable<T> | string, previously T | string}functionerror(): never{thrownewError();}functionf2<T>(x: T){// Now NonNullable<T>, previously Treturnx||error();}functionf3(x: unknown){lety=x!;// {}}functionf4<Textends{x: string|number}|undefined>(obj: T){if(obj?.x==="hello"){obj;// Now NonNullable<T>, previously T}if(obj?.x){obj;// Now NonNullable<T>, previously T}if(typeofobj?.x==="string"){obj;// Now NonNullable<T>, previously T}}

[Daniel Rosenwasser (DanielRosenwasser)]

FWIW this added about 100ms to Compiler-Unions