Add collision check for 'Reflect' when using super in static initializers

[rbuckton]

This adds a missing collision check for the global Reflect object when encountering a super reference in a static initializer in ES2015-ES2021.

The collision check isn't needed in <=ES5 for the following reasons:

• We don't support uncalled SuperProperty access in <=ES5
• We use _super in <= ES5
• Reflect isn't available in ES5, so we do not use it as part of the transformation for targets below ES2015.

This also updates needCollisionCheckForIdentifier to avoid false positives for type-only imports.

[sandersn]

One initial question and one suggestion. I'll look at the rest of checker.ts' changes tomorrow.

[sandersn]

Looks good, just a few small questions/formatting.