This CL adds support for macros, builtins, generics and specializations
for the "textDocument/symbol" request. To filter out implicitly
created specializations, the "is_user_defined" flag is hoisted from
Macro to the Declarable super class. As a side-effect, errors thrown
during specialization now have the correct SourcePosition.

Drive-by-change: Using "Goto Definition" on the identifier of the
specialization will jump to the associated generic.

Bug: v8:8880
Change-Id: I0c60571c58107375c1b5d2a8e620cf12a0f0f3fc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1609795
Commit-Queue: Simon Zünd <szuend@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61486}