Optimize source mapping into external source map sources

[JoostK]

Fixes #40054

Timings

fn	Before	After	Improvement
emit phase	13.4	8.8	-34%
emitSourcePos	4.8s	0.6s	-87%
setSourceMapSource	4.3s	0.1s	-98%

[JoostK]

Implementation note: this could alternatively be done without a full cache, using just a single MRU fileName/sourceIndex. That does however require that the second call to setSourceMapSource in emitSourcePos has to take an additional parameter to not even call addSource (which is safe as reverting to the prior source map source is known to have been added) as otherwise 2 MRU items are needed:

TypeScript/src/compiler/emitter.ts

Lines 5290 to 5300 in a7d8011

	function emitSourcePos(source: SourceMapSource, pos: number) {
	if (source !== sourceMapSource) {
	const savedSourceMapSource = sourceMapSource;
	setSourceMapSource(source);
	emitPos(pos);
	setSourceMapSource(savedSourceMapSource);
	}
	else {
	emitPos(pos);
	}
	}

This alternative would also mitigate the issue (I verified that) as it's typical for a single external source map source to be applied on nodes without interference from nodes that have a different external source map source, so a single MRU cache entry is sufficient.

[DanielRosenwasser]

Shucks, we really should have run a perf test on this.

@typescript-bot perf test this

[typescript-bot]

Heya @DanielRosenwasser, I've started to run the perf test suite on this PR at 4b4a156. You can monitor the build here.

[DanielRosenwasser]

well duh, that'll do nothing now.