This CL introduces a declaration_position_ field on TypeAlias,
corresponding with the SourcePosition of the name of the
Type where it is declared.
This information is needed by the language server for
"goto defintion".

R=tebbi@chromium.org

Bug: v8:7793
Change-Id: I0de2f7b7ba23b86de34441107ca9982d190c227f
Reviewed-on: https://chromium-review.googlesource.com/c/1497952
Commit-Queue: Simon Zünd <szuend@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60008}