[LLD][COFF] Skip computation of the undefined symbols references that are not shown

The "undefined symbol" error message from lld-link displays up to 3 references to that symbol, and the number of extra references not shown.

This patch removes the computation of the strings for those extra references.

It fixes a freeze of lld-link we accidentally encountered when activating asan on a large project, without linking with the asan library.
In that case, __asan_report_load8 was referenced more than 2 million times, causing the computation of that many display strings, of which only 3 were used.

Differential Revision: https://reviews.llvm.org/D83510