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Closes #31862.

1. we never disposed tag aggregator leaking all taggers ever created on the buffer involved in contained document

2. we hold on to taggers not related to our buffer but ones that are related to other buffers. for example, our subject buffer is actually subject buffer which involves template C# buffer + html buffer where portion of html buffer are mapped to the subject buffer. that we end up holding on to all html taggers.

Fixes an occasional failure in the ErrorList test.

Fix null check combined with nullable bool test

Pattern-based 'await foreach' should find parameterless 'GetAsyncEnumerator' and 'MoveNextAsync' (#32184)

Fix IDE0055 formatting errors

* Run CI builds for 16.0 preview2 vs-deps PRs

* Trigger branch builds on commit for preview branches

Fixup the triggers

Fixup our triggers for branches.

Publish VSIX installer with build

Change no conversion to idisposable message

Fix parsing regression on switch case expression that is parenthesized unary expression

- Restore previous message to remove any mention of 'public void-returning Dispose method'
- Update test comments to match

Merge Features/enhanced-using to master

Fix async test

Remove prototype comments

- Stop classifying EndKeyword in EndUsingStatement
- Begin classifying ToKeyword in ForStatement
- Begin classifying IfKeyword in TernaryConditionalExpression

Fixes #32161