Compiler service: Non-deterministic typar naming. #14850

The skipImplFilesIfSigExists feature introduces non-deterministic naming of type parameters.
This is due to race conditions triggered by breaking the otherwise fully sequential nature of type-checking.

Repro steps

Provide the steps required to reproduce the problem:

1. Open the following code in Rider: NondeterminismSample.zip
2. Open the 5 files one-by-one in compilation order, wait for code analysis to finish.
3. In NonGenericUse.fs hover over the A.ReportWarnings call. You should see that the type parameter's name is 'b:

Here is a gif showing the steps 1-6 performed:

Expected behavior

The name of the type parameter should not depend on the order of files being opened.
As noted in #14836 (comment) , the expectation is that typars would not leak between files, so their names should be established within the same file they are defined in.

Actual behavior

There is a race condition. Depending on user actions and on which callsite is visited first, the typar gets a different name assigned.

Known workarounds

• Do not use the skipImplFilesIfSigExists feature. .fsi files have been marketed for quite some time as a way to speedup IDE performance, so having to disable this feature would be unfortunate.
• Ignore the discrepancy. This would probably be fine, except that the issue surfaces a more general problem that skipImplFilesIfSigExists generates - mutable parts of TcState are reused between different files' type-checks and independent type-checking requests affect each other's results.

Related information

Provide any related information (optional):

• Editing Tools - Rider

Note that the same problem can be seen in compilation when any sort of parallel type-checking is attempted: #14734 .
The underlying problem is the shared, mutable state, causing race conditions.