[LLD] [COFF] Use StringTableBuilder to optimize the string table

This does tail merging (and deduplication) of the strings.

On a statically linked clang.exe, this shrinks the ~17 MB string
table by around 0.5 MB. This adds ~160 ms to the linking time
which originally was around 950 ms.

For cases where `-debug:symtab` or `-debug:dwarf` isn't set, the
string table is only used for long section names, where this
shouldn't make any difference at all.

Differential Revision: https://reviews.llvm.org/D120677