This CL changes a few AST nodes and one internal structure to use "Identifier"
instead of a simple std::string, allowing the use of the exact source position
of such a name.

Drive-by change: Add new constructor to Binding that allows creation from
an Identifier. It sets the declaration_position to the source position of
the name instead of using "CurrentSourcePosition".

R=tebbi@chromium.org

Bug: v8:7793
Change-Id: I456d20822b6528d2e96400c28ebfebea3ca80613
Reviewed-on: https://chromium-review.googlesource.com/c/1491223
Commit-Queue: Simon Zünd <szuend@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59899}