Fixes #19926

Fixes #19927

Remove IUnboundLambdaExpression from IOperation API, return an ILambdaExpression for UnboundLambda nodes.

Fixes #19902

The PDB path which is written into the PE file should respect the
`/pathmap` option passed to the compiler.  This is necessary to ensure
that PEs can be deterministic when built from different source paths.

The feature flag `pdb-path-determinism` is being kept for the time
being.  Even though it's a feature flag it still seems inappropriate to
break customers in a point release.  Deferring the removal until the
next major release to give customers time to react.  This issue tracks
removing the flag:

https://github.com/dotnet/roslyn/issues/19592

closes #9813

Fixes #18300

A single method could have spans in multiple source files. For example, synthesized constructors of partial types can have spans in multiple files if this type has field initializers in multiple files. Another example is methods with #line directives.

This change changes instrumentation for such methods - the instrumented IL for such methods will now include a call to a new overload of Microsoft.CodeAnalysis.Runtime.Instrumentation.CreatePayload() that accepts an array of file indices as opposed to single file index.

* simplify partial method return type syntax finder tests

* Bug fixes and API use changes

* address first review

* better comment and compiler trait on class

* fix tests, pull entrypoint check out into local function

* unify diagnostics for intvoid or task mains

* add more tests, change error text

* address review

* finish tests

* be explicit on language version in tests

* add another test, fix typo, add diagnostics even when they aren't likely to exist

* rewrite error->warning code, add more cases to tests

* change test language versions

* formatting on comments and adding WithLocation to tests

* fix formatting for some tests

* fix some formatting and added withlocation

* fix withlocation

There are couple of changes here:
1. API change: `ImmutableArray<ISymbolInitializer> MemberInitializers` is changed to `ImmutableArray<IOperation> Initializers`.
2. Implementation changes:
   1. Instead of returning the member initializers as synthesized ISymbolInitializer nodes, we now return member intializers as IAssignmentExpression nodes. This ensures completeness of IOperation tree.
   2. Now we also return the collection intializer expressions within an object creation expression.

Fixes #18115

There are 2 bugs still affecting this area:
1. #18781: IOperation API shape for collection initializer expressions
2. #19276: Missing Field/Property reference expression nodes in object creation initializer node

This is an automated migration of our project files to the new SDK /
PackageReference.  It is done using the tool located here:

https://github.com/jaredpar/ConvertPackageRef

* attempt to get main entrypoints working

* async main codegen

* remove garbage file

* fix vb tests

* add some comments

* moduleBeingBuilt can sometimes be null apparently

* fix style nits

* more whitespace fixes

* address feedback

* automated style fixer

* move most work out into the constructor

* protect against null in error condition

* add comment

* address cston review

* autoformat

* use cast

* check module being built

* add another object cast

* address review

* check for old cases where async methods called Main also exist

* more suggestions

* autoformat

* mostly cleanup

* use shorter Count and Single

* remove superfluous diagnostics add

* check exit code / revert to old entrypoint finding code

* remove language version check in entrypoint compiler

* fix debug.assert

* switch to assert equal

* codereview

* add more tests; change conditions for test execution

* fix up emit methods

* narrow down error location

* ammend GetAwaitableExpressionInfo to return a bound call to GetAwaiter().GetResult()

* more comments

* find entrypoint methods uses the async binder

* autoformat

* use diagnostics from async binder

* relocate tests

* fix suggestions for review 8346

* add back no-async tests with better location

* only allow named types

* remove empty file; remove null checks from helper

* forgot to add csproj file

* emit into warning diagnostics instead of regular diagnostics

* autoformat

* remove unused file from merge

* use diagnostic locations

* move some methods around, split some tests up

* fix non-returning async tests, remove bool return from check availability

* add params test

* style and expected output fix

* switch back to null because default emits warnings

* add partial method tests

* fix locations for unix

* better partial syntax tests

* use expected length of 0

* use default length of 0 for expected output

* check implementation symbol