More build changes

Add feature that offers to convert from a conditional expression to CoalesceExpression.

This change visits the parameters before "replaying" reads+writes from
calls of local functions for the purposes of definite assignment. This
ordering is observable because a variable can be definitely assigned in
the argument to the call, and then used inside the call.

Fixes #14243

Update our internal publishing

Adjust the way parser handles line-If termination.

This changes our generic-lowering logic to use 'Arity' instead of
'IsGenericType' to figure out whether or not a local function frame
needs generic subsitution. We previously used IsGenericType, which is
inappropriate because it also considers whether or not the containing
type of the frame is generic. The containing type is necessarily already
subsituted when calling a local function, so it will never be possible
to substite type parameters.

Instead, we should use Arity of the frame, which is only non-zero if the
frame itself is generic, regardless of the containing type.

Change our internal publishing to use the new NuGet server vs. the old file share

This follows standard MSBuild conventions for file extensions:

- props: imported at the start of projects.  Primarily for setting properties.
- targets: imported at the end of projects.

Build changes

Update tests to use multiline literals.

Fixes #14785

Fixes #14825

Merge dev15-rc into master

Fixing up our targets / settings file

Also clean up some grammar in Syntax.xml
Fixes #14651

Fix root path being truncated when formatting errors

Fixes #14761.