This CL introduces a new expression that can replace "std::string"
fields in other expressions. The main goal is to get SourcePositions
for identifiers to make them available in the language server.

The CL introduces a separate symbol "name", that allows to
incrementellay replace strings with name expression where needed. As
an example, variable delcarations now use a NameExpression for the
variable name.

R=danno@chromium.org, tebbi@chromium.org

Bug: v8:7793
Change-Id: I5b88bbaeac597b8e9760d2e01880e5e599ebf802
Reviewed-on: https://chromium-review.googlesource.com/c/1488752
Commit-Queue: Simon Zünd <szuend@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59865}