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

[Martin Storsjö via llvm-commits]

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@@ -864,9 +865,9 @@ void LinkerDriver::addWinSysRootLibSearchPaths() {
 
   // Libraries specified by `/nodefaultlib:` may not be found in incomplete
   // search paths before lld infers a machine type from input files.
-  std::set<std::string> noDefaultLibs;
-  for (const std::string &path : ctx.config.noDefaultLibs)
-    noDefaultLibs.insert(findLib(path).lower());
+  llvm::StringSet<> noDefaultLibs;
+  for (auto &iter : ctx.config.noDefaultLibs)
+    noDefaultLibs.insert(findLib(iter.first()).lower());
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mstorsjo wrote:

Right, that's quite possible. But I don't want to take AI's word for it - can you look up somewhere in `StringSet`/`StringMap` which says or shows this?

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