Move to newer version of Microsoft.Net.Compilers and Microsoft.NetCore.Compilers compatible with latest analyzers package

The [change](https://github.com/dotnet/roslyn/commit/6be56fadb4c417c4b6af0bf62f1242e41c8dd124) to move analyzers to 2.5 version broke analyzers in the branch - they are laid out differently in the new nuget package. Build targets are unable to find the props for importing the analyzers and silently bail out executing them. This PR fixes the following:

1. Fix the props file path in Imports.Targets
2. Move to a newer analyzer package which has an important [bug fix](https://github.com/dotnet/roslyn-analyzers/pull/1255)
3. Report a build error if analyzer props file does not exist
4. Fix the public API errors (mostly due to duplicate entries in shipped and unshipped files).
5. Fix one instance of RS0014 (Do not use Enumerable methods on indexable collections. Instead use the collection directly) introduced recently in the code.
6. Add suppression for one instance of CA1802 in WellKnownAttributeData.cs that fixed the signing issue.

Fixes #18985

Report progress as we update the package index.

Merge dev15.3.x to master

Remove dead code from GenerateConstructor.

# Conflicts:
#	build/Targets/Imports.targets
#	src/EditorFeatures/Core/EditorFeatures.csproj

Initialize snippets after solution load is complete

Show modifier/method/type block declaration keywords after attribute …

Disable NETCore version check

Fixed smart variable naming

Works around https://github.com/dotnet/roslyn/issues/20945

Disable NETCore version check

Diagnostic analyzer to identify out of range placeholders in a format string.

Works around https://github.com/dotnet/roslyn/issues/20945

Fixes https://github.com/dotnet/roslyn/issues/20837

Wait for encapsulate field to complete

Check for prefix match before hard selecting when backspacing

Add an analyzer that offers to convert from a lambda to a local function.

* Improve type argument list parsing in error conditions.

* Fix type.

* Remove unused method.

* Improve generic parsing in a couple of additional cases.

* Share more code.

* Inline method.

* Fix logic.

* Add tests.

* Restore test class.

* REmove case, and add test.

Ensure we disconnect any projection buffers we create for views

Add null check in ExplicitInterfaceTypeCompletionProvider for invalid code

* Don't report modifier errors while parsing.

* Fix bad merge.

* restore code.

* Fix test.

* Remove old resource

* Alternative way to do partial checking.

* Alternative way to do partial checking.

* Simplify formatting.

* Restore formatting.

* Restore formatting.

* Restore formatting.

* Restore formatting.

* Restore formatting.

* Give a proper message.

* Update error positions.

* Update error positions.

* Update error positions.

* Actually report error on bad modifier.

* 	Simplify implementation of parsing constraints.

* Add tests.

Fixed Inline Temporary Variable refactoring

Swaps out all uses of CapturedVariablesByLambda for functions on the
Scope tree.

The reason the baseline changed is that the order of enumerating closures changed. I briefly looked into matching the behavior, but the problem is that the closure order in the old visitors is determined by the visitation of captured variables, not visitation of the closures themselves. More specifically, an item is only added to the CapturedVariablesByLambda dictionary when ReferenceVariable encounters a captured variable.

In contrast, the visitation of the Scope tree looks at closures in an in-order traversal of the tree, stopping at the first introduced closure, not necessarily the closure which first captured a variable.

One obvious consequence of this changing is that the old ordering for a series of nested lambdas was effectively post-order -- the interior closures would be added to the visitation first, since they would be added from the perspective of the captured variable looking up, instead of the visitor looking down. This particular case is responsible for all the baseline changes that I have seen.

When I looked into replicating this traversal order I found it complicated and pretty counter-intuitive. Since the baseline change was so low in emitted IL, I thought it better to keep a very natural in-order tree traversal and just rebaseline a few tests.

Fixes https://github.com/dotnet/roslyn/projects/26#card-3753318