[LLD] [COFF] Fix symbol names for import thunks (#160694)
9cc9efc483339ece1d52923569bb755db42b69f3 changed LLD to use a
StringTableBuilder for optimizing the string table, used for long
section and symbol names.
That commit had a bug, where the symbol table entry for an import thunk
with a long symbol name wouldn't get fetched from the
StringTableBuilder, if the base symbol name (without the "__imp_"
prefix) wasn't over 8 chars.
This should fix issues with Go, which errors out on reading the
executables with a broken symbol table, as noted in
https://github.com/mstorsjo/llvm-mingw/issues/518 and
https://github.com/golang/go/issues/75219.
