Improve the performance of updateExportedFilesMapFromCache

[amcasey]

#45238 mitigates performance problems in updateExportedFilesMapFromCache but doesn't address the underlying problems.

1. Why is updateExportedFilesMapFromCache called so many times during the course of building a single projects?
2. If not making any updates is the common case, would another representation be more appropriate (e.g. storing a delta, rather than a copy)?
3. If we continue to update the "real" map from a full cache, can the update share code with clone? (The PR doesn't because clone appears to make a shallow copy which (a) seems wrong and (b) wasn't what the code being replaced did.)

[sheetalkamat]

clone appears to make a shallow copy

This was intentional.. After state is formed, the referenceMap (the way it was normal map before) can only be set with New set of values.. doesnt modify existing values..

[andrewbranch]

@sheetalkamat @amcasey is this issue still actionable? I noticed #45238 got reverted in #48579.

[sheetalkamat]

Fixed by #48579