[MS.Build] output item was dropping metadata after running task.
[MS.Build] add another expression expansion test.
[MS.Build] remove extraneous expression evaluation code.
[MS.Build] more of the same (remove obsolete comment).
[msbuild] ongoing ITaskItem input parameter handling improvements.

Right now all the input parameters are computed based on string, and any
metadata items are simply chopped out when it is passed to a Task.
To fix this, we need to replace ExpandString() use with more complicated
evaluator that takes ITaskItem into consideration.

(It is not complete as of now; evaluation method needs to be replaced and
any regressions need to be fixed.)
[MS.Build] skip null string entry as input to string.Join().
[MS.Build] now ITaskItems are collected while evaluating expression and metadata is filled later.

It is kind of reverting previous implementation of item metadata for expression.
Code cleanup should follow.
[MS.Build] cleanup ITaskItem evaluation related old code.