InlineRefactor: rename inlExpLst to InlineContext

InlineRefactor: rename inlExpLst to InlineContext

Rename inlExpLst to InlineContext, and move the code for it into
the inlining files. Expand the context to capture the observation
leading to success. Defer generating the method names until the
context is dumped to avoid unnecessary debug/check allocations.

Make sure observation field on GenTreeCall is properly initialized
and copied, and give it the right type via an opaque forward
declaration.

Commit migrated from https://github.com/dotnet/coreclr/commit/c18892218e6b302eaaf504945feb5f469f65f663