Remove the function identifier field from SharedFunctionInfo. This field
would store one of a) the function's inferred name, b) the "builtin
function id", or c) debug info. We remove these in turn:

a) The function's inferred name is available on the ScopeInfo, so like
   the start/end position we read it off either the ScopeInfo (for
   compiled functions) or the UncompiledData (for uncompiled functions).

   As a side-effect, now both UncompiledData and its subclass,
   UncompiledDataWithPreparsedScope, contain a pointer field. To keep
   BodyDescriptors manageable, we introduce a SubclassBodyDescriptor
   which effectively appends two BodyDescriptors together.

b) The builtin function id is < 255, so we can steal a byte from
   expected no. of properies (also <255) and store these together.
   Eventually we want to get rid of this field and use the builtin ID,
   but this is pending JS builtin removal.

   As a side-effect, BuiltinFunctionId becomes an enum class (for better
   storage size guarantees).

c) The debug info can hang off anything (since it stores the field it
   replaces), so we can attach it to the script field instead.

This saves a word on compiled function (uncompiled functions
unfortunately still have to store it in UncompiledData).

Bug: chromium:818642
Change-Id: I8b4b3a070f0fe328aafcaeac58842d144d12d996
Reviewed-on: https://chromium-review.googlesource.com/1138328Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54543}