[llvm-objcopy] Improve performance of long pattern lists

Some users use a long list of fixed patterns (PR50404) and
O(|patterns|*|symbols|) can be too slow. Such usage typically does not use
--regex or --wildcard. We can use a DenseSet<CachedHashStringRef> to optimize
name lookups.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D105218