Refactoring: Make method async and await call-sites

Is your feature request related to a problem? Please describe.

Migrating a codebase to use async overloads where available and make the enclosing methods async as well.

Describe the solution you'd like

Whenever an async overload is available in a non-async method, suggest to use that, make method async and update call-sites.

If this is done as a fix-all, this can be recursively applied to the call graph.

Describe alternatives you've considered

Currently this can be done in a few repetitive steps until the whole thing is migrated, but it is extremely cumbersome:

1. Change some method call to Async overload to get VSTHRD110, or
   Change method return type to Task to get VSTHRD103 (note: 'change signature' doesn't support this)
2. Use 'Add await' codefix to get CS4033
3. Use 'Make method async' to get VSTHRD110 or CS4014 on call-sites
4. Repeat

Start by tracing the existing VSTHRD103/VSTHRD110 diagnostics and the Add await and Make method async code fixes described in the issue. Map how a changed method signature propagates to call-sites, then define bounded fix-all behavior; done means async overloads, enclosing methods, and call-sites can be migrated consistently.