Adds a (currently unused) constant_pool() field to BytecodeArray objects.
This field points to a FixedArray object which will be used to hold constants.

The BytecodeArray is now a mixed values object type, with the
kConstantPoolOffset object holding a tagged pointer, but the remainder of the
object holding raw bytes (which could look like tagged pointers but are not).
Modify the BytecodeArray GC visitors to deal with this and test that the
field is migrated properly when evacuated.

BUG=v8:4280
LOG=N

Review URL: https://codereview.chromium.org/1314953004

Cr-Commit-Position: refs/heads/master@{#30404}