[lld] [lld][COFF] Use `.contains` rather than `.count` for set membership. NFC (PR #177067)

[Martin Storsjö via llvm-commits]

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@@ -155,14 +155,14 @@ struct Configuration {
   // Symbols in this set are considered as live by the garbage collector.
   std::vector<Symbol *> gcroot;
 
-  std::set<std::string> noDefaultLibs;
+  llvm::StringSet<> noDefaultLibs;
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mstorsjo wrote:

Note that this change isn't a no-op in itself; a `std::set<std::string>` owns the storage of the strings, while `StringSet` presumably(?) only uses `StringRef` which just references strings whose storage is handled elsewhere.

https://github.com/llvm/llvm-project/pull/177067