JS: Make SourceNode and PropRef.getBase non-recursive, again

[asgerf]

• Makes SourceNode non-recursive (really, this time)
• Makes PropRef.getBase non-recursive by fixing an accidental reference to SourceNode
• Caches PropRef.getBase

Although #5499 was supposed to make SourceNode non-recursive, I had missed one recursive dependency going through SourceNode -> AngularJS -> Import -> NodeJS/createRequire -> SourceNode. My earlier attempt in #4772 fixed that by making all string literals SourceNode but that was a little too costly, so now I've instead made createRequire not depend on SourceNode.

Evaluation (internal link) looks good; an average 1% speed-up and a <1% increase is cache size.

[asgerf]

@esbena I managed to make a non-recursion test that should work well in practice by requiring that SourceNode::Range cannot depend on SourceNode. In theory it's still possible for SourceNode::Range to be recursive by itself, but in practice any accidental recursion will go through SourceNode so we should have a good chance of catching it now.

[esbena]

LGTM.
It is a shame that we can't easily add a failing recursion guard test to ensure that the compiler does not get smart enough that it bypasses the test one day.