The initial implementation of {FixedSizeSignature} contains undefined
behaviour, because {InitReps} wrote to the {reps_} array before the
constructor of that array has been called.
This also resulted in bugs if {FixedSizeSignature} was used with types
that actually have a constructor (like {ValueType}). The array
constructor would call the default constructor on each contained
element, thus overwriting the values written by {InitReps}.

This CL fixes that by switching to a plain array, and only writing to
the array in the body of the constructor (after the field was properly
initialized).

It also removes the {Concat} method in favor or simply copying from two
input arrays in a private constructor.

Drive-by: Use proper constant names for the template parameters to
make cpplint happy.

R=ahaas@chromium.org

Bug: v8:11384
Change-Id: Id748c8fef3c846069f91843f74d0555ed8ca9fb7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2773799Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73540}