[clangd] Don't clone SymbolSlab::Builder arenas when finalizing.

SymbolSlab::Builder has an arena to store strings of owned symbols, and
deduplicates them. build() copies all the strings and deduplicates them again!
This is potentially useful: we may have overwritten a symbol and
rendered some strings unreachable.

However in practice this is not the case. When testing on a variety of
files in LLVM (e.g. SemaExpr.cpp), the strings for the full preamble
index are 3MB and shrink by 0.4% (12KB). For comparison the serializde
preamble is >50MB.
There are also hundreds of smaller slabs (file sharding) that do not shrink at
all.

CPU time spent on this is significant (something like 3-5% of buildPreamble).
We're better off not bothering.

Differential Revision: https://reviews.llvm.org/D135231