GeezerVison

Fixes #47146 (introduced by #42425 which enabled this syntax)

Previously this code ran without error:

type OH_NO = {
  get [K in WAT](): string
};

because we incorrectly swallowed the errors in checkComputedPropertyName under the assumption that we were looking at a { [K in T]: U } declaration. The code path where the error would have been detected -- in the mapped checking logic -- obviously does not run because this is not actually a mapped type.

Thankfully the computed type was completely useless (see comments in linked issue), so it isn't feasible that anyone took a dependency on this.