[dsymutil][DWARFLinker][NFC] Refactor usages of UniquingStringPool.

That refactoring is helpful since it reduces data inter-dependencies.
Which is good for current implementation and even more good for
fully multi-thread implementation. The idea of the refactoring
is to delete UniquingStringPool from the global DWARFLinker level.
It is used to unique type names while ODR deduplication is done.
Thus we move UniquingStringPool into the DeclContextTree which
matched to UniquingStringPool usage scope.

golden-dsymutil/dsymutil 23787992
clang MD5: 7d9873ff94f0246b6ab1ec3e8d0f3f06

build-Release/bin/dsymutil 23921272
clang MD5: 7d9873ff94f0246b6ab1ec3e8d0f3f06

Differential Revision: https://reviews.llvm.org/D93460