Use a frozendictionary on net-core and a readonly one on netfx

[CyrusNajmabadi]

Running a test insertion to see if this removes the recent regressions caused by: #72965

PR1: https://devdiv.visualstudio.com/DefaultCollection/DevDiv/_build/results?buildId=9406987&view=results
PR2: https://devdiv.visualstudio.com/DefaultCollection/DevDiv/_build/results?buildId=9407001&view=results

FrozenDicts turn out to be quite expensive on NetFx if you're using StringComparer.CaseInsensitive. It causes thsi sort of blowup as the dict analyzes keys:

This issue is not present on netcore, where FrozenDict can do all of this work in a non-allocating fashion.

[ToddGrun]

Post here when you get results from the test insertions, as I'm curious whether this improved things

[CyrusNajmabadi]

Will do.

[ToddGrun]

Given stephen's change, this feels slightly less appealing even if the speedometer test results come back with good results

[CyrusNajmabadi]

Note: we'd have to wait for stephen's change to make it all the way out into a release that we can pull into VS. When that happens, i'll undo this an dmove to FrozenDict uniformly.

[CyrusNajmabadi]

Perf tests are here https://dev.azure.com/devdiv/DevDiv/_git/VS/pullrequest/543655

They show that there's no regression at all now that we use a RODict in netfx and a FrozenDict in core:

[ToddGrun]

When that happens, i'll undo this an dmove to FrozenDict uniformly.

Might be nice to slip in a TODO comment in one of your upcoming PRs that this is temporary and a link to Stephen's PR

[CyrusNajmabadi]

I simply will not forget. With my memory being perfect, there's no chance of anything going wrong.

[CyrusNajmabadi]

@jasonmalinowski For review when you get back.