We already had this inside of VisualStudioRuleSetManager, and we need
it for more things too. For now all the locking semantics are being
kept identical.

The basic mental here is to determine a context for a file, we start
with the Running Document Table and grab the IVsHierarchy for that file.
We then chase it through shared projects until we arrive at final
IVsHierarchy. This should be the IVsHierarchy we were given for the
project. In the case of multitargeting projects, there's one last hop
where we fetch a VSHPROPID_ActiveIntellisenseProjectContext that just
returns the project name instead of a IVsHierarchy. If you squint you
could imagine that as if it returned the IVsHierarchy for that project
except we don't have unique ones.

To set the context, we start with the project and walk those
IVsHierarchy pointers backwards. At each step we can do an operation to
figure out which IVsHierarchy needs its pointer updated, and we point it
to build the chain, moving back to the IVsHierarchy that will end up
in the Running Document Table.

This is (still) leaking IVsHierarchies that we subscribe to, and that
will be fixed next.

Avoid navigating to message severity items unless requested

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