Make sure nullability mismatch in constraints specified in different partial declarations (types/methods) are properly detected and reported. (#35272)

 Make sure nullability mismatch in constraints specified in different partial declarations (types/methods) are properly detected and reported.

Fixes #30229.
Fixes #35179.

Implements the following LDM decision:

For partial types, the invariant matching from type inference and merging. A mismatch
between two non-oblivious candidates produces an error. No warnings are produced.

For partial methods, nullability has to match with exception for oblivious and we produce warnings.
For the result, we use the implementation signature inside the implementation, and the
declaration signature for the callers.

Move to non Int pools

Bugfix fix: "Replace method with property" removes a space

Add new optprof test for training

Responding to a request from our core engineering team to move to a
different pool.

DynamicFileInfo.cs: fix typo in the constructor

Merge dev16.1 to master

VS 2017 -> VS 2019

Fix applying code actions that change AdditionalDocuments

Some code was accidentally calling GetDocument with an additional
document ID; this didn't end well.

give CSharpAsAndNullCheckCodeFixProvider more intelligence about removing statements

Merge dev16.1 to master

More Pull Members Up MAS Fixes

* Make Generated syntax trees restore to project nullability

Fixes #29894

A reference to an unconstrained type parameter in nullable-disabled code should be treated as *oblivious*, and therefore reading them should be considered to produce non-null values, and we are permitted to assign null values to them without a diagnostic.
Fixes #34842

Also disable the old WRN_DotOnDefault when the nullable feature is enabled.
Fixes #34855

Ensure we refresh ruleset severities after a ruleset change

Merge dev16.1 to master

When we relaxed the requirement for pattern matching open types to a
constant pattern to not require a conversion from the pattern expression
to the open type, but the pattern expression should be required to have
a constant value.

Fixes #34980

Merge dev16.1 to master [SHA-only]

* Implement MetadataAsSource for ref and readonly structs

* MetadataAsSource for readonly members

* Fix NotImplementedException errors. Fix some ArrayBuilder leaks.

* Fix a leak. Fix implicit readonly attribute test.

* List passed to GetUpdatedDeclarationAccessibilityModifiers needs to support Remove

* Allow specifying a metadata language version for MetadataAsSourceTests

* Remove unused SyntaxTokenList members. Comment readonly event generation.

* Check VB MetadataAsSource for readonly members

* Fixes from feedback

* Add test for not implicitly readonly struct getter

* Label arguments in call to GetCustomAttributesForToken

* Reference issue about readonly event public API

* intellisense should suggest event after writing readonly in a struct member declaration.

* Avoid repetition of keywords in EventKeywordRecommender

Merge dev16.1-preview3 to dev16.1 [infra only]

Using FQN instead of adding import during ENC session

Don't provide sync namespace refactoring in generated code

Fix exception in PasteTrackingService.RegisterPastedTextSpan

Fix `nameof(MethodName)` not classifying MethodName

Update azure-pipelines* for dev16.1-preview3*

Update PublishData for 16.1 preview3 and preview4

Update versions.props for 16.1 preview 4

The current implementation of closure conversion creates closure environments for each
new scope. This change tries to minimize the number of closure environments, and thus
closure environment allocations, when possible by merging "adjacent" closure environments.

To merge two closure environments, they have to:

1. Be captured by exactly the same set of closures
2. Have no backwards branching between the allocation of the two environments
3. Have the same environment lifetime

If so, all of the variables will be merged into a single closure environment. 

Fixes #29965