Fix 'keyof' for constrained type parameters

[ahejlsberg]

This PR introduces two assignability rules to properly support constrained type parameters in conjunction with the keyof type operator:

• Given a type parameter K with a constraint keyof T, a type S is assignable to K if S is assignable to keyof T.
• Given a type parameter T with a constraint C, a type S is assignable to keyof T if S is assignable to keyof C.

Fixes #12011.

[DanielRosenwasser]

Linting errors!

10:14:39 src/compiler/checker.ts[6831, 25]: Identifier 'constraint' never appears on the LHS of an assignment - use const instead of let for its declaration.
10:14:39 src/compiler/checker.ts[6841, 25]: Identifier 'constraint' never appears on the LHS of an assignment - use const instead of let for its declaration.
10:14:46 scripts/tslint/preferConstRule.ts[5, 186]: Unnecessary semicolon
10:14:49 scripts/tslint/booleanTriviaRule.ts[5, 143]: Unnecessary semicolon

[DanielRosenwasser]

Otherwise looks good to me. 👍