Spelling fixes

Merge release/dev16.4-preview1 to master

Spelling fixes

Spelling fix

* x == default is valid object equality for reference types
* Block null == default

Spelling fix

Spelling fix

Spelling fix

Adds the definitions for the nullable attributes and creates an
annotated Debug.Assert

Update README.md

Update AbstractAddAccessibilityModifiersCodeFixProvider.cs

Update SyntaxGeneratorTests.cs

Update TypeImportCompletionItem.cs

A common case for editorconfig is for most of the source files to have the same
options specified, which means if we store options for each source file it's
likely we'll have many duplicate sets. This change adds a cache to AnalyzerConfigSet
to try to de-dup identical options dictionaries to a single shared instance.

Update utils.ps1

Spelling fix

Update required version of VS in Building doc

Change icon for target type completion filter

Merge dev16.4 preview1-vs-deps into dev16.4 preview1

Fix an RPS regression where we are eagerly loading all assemblies wit…

Fix an RPS regression where we are eagerly loading all assemblies with ITableColumnDefinition implementations

Recent change #37802 to enable new containing type and containing member colums removed the perf optimization done in #36562 to ensure that we do not eagerly load all assemblies that contain ITableColumnDefinition implementations. This caused an RPS regression, which is fixed by this PR.

Properly pass along actual referenced symbol in FindCallersAsync result.

Merge pull request #38814 from dotnet/merges/release/dev16.4-preview1-to-release/dev16.4-preview1-vs-deps

Merge release/dev16.4-preview1 to release/dev16.4-preview1-vs-deps