We can save a pointer of space for each CodeEntry by removing this
field which we don't really need. Instead of concatenating the name
string on demand, concatenate the prefix eagerly.

Reduces sizeof(CodeEntry) from 136 to 128 on 64-bit.

Bug: v8:7719

Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: Id346a8f36794e337e8c886f8d1969431424539b0
Reviewed-on: https://chromium-review.googlesource.com/1039825Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Alexei Filippov <alph@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53014}