Add the ability for the typer to track whether a string could be the empty
string. This is needed for typed lowering of JSStringConcat since we can't
create cons string chain with the empty string in arbitrary positions.

The ToPrimitiveToString bytecode handler is modified to collect feedback on
whether it has ever seen the empty string, which is used by
SpeculativeToPrimitiveToString to ensure that the output is non-empty (or
depot) which will subsiquently be used to enable inline cons-string creation
for the JSStringConcat operator in typed lowering in a subsiquent CL.

BUG=v8:6243

Change-Id: I41b99b59798993f756aada8cff90fb137d65ea52
Reviewed-on: https://chromium-review.googlesource.com/522122
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45786}