Further possible fix to reduce memory usage

[dsyme]

@TIHan and others have noticed excessive creation of ILModuleReader. Here's a second possible fix, to be evaluated

The first possible fix is at #6389

[dsyme]

@KevinRansom I prefer this one to #6389

@TIHan What do you think?

@davidkean may also want to review

[KevinRansom]

Thanks

[brettfo]

FYI, I just had to revert this in #6468 due to a build failure in master (link in revert PR).

[KevinRansom]

Pr to fix build issue for #648:

[dsyme]

@auduchinok asked about this. The use of two caches is explained by these two comments:

// Cache to extend the lifetime of a limited number of readers that are otherwise eligible for GC
...
// Cache to reuse readers that have already been created and are not yet GC'd

[TIHan]

I have evaluated this change and with the current internal project that I've been testing, this significantly reduced memory consumption after a find all refs. Before, we would reach 2.6gb in VS process, and now it hits 1.9gb specifically with this change.

This is fantastic.

I was wondering why 16.1 was not hitting the 2.6gb limit.