[clang] [llvm] [Clang] Match MSVC handling of duplicate header search paths in Microsoft compatibility modes. (PR #105738)

[Shafik Yaghmour via cfe-commits]

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@@ -3190,19 +3190,27 @@ static void GenerateHeaderSearchArgs(const HeaderSearchOptions &Opts,
   auto It = Opts.UserEntries.begin();
   auto End = Opts.UserEntries.end();
 
-  // Add -I... and -F... options in order.
-  for (; It < End && Matches(*It, {frontend::Angled}, std::nullopt, true);
+  // Add the -I..., -F..., and -iexternal options in order.
+  for (; It < End && Matches(*It, {frontend::Angled, frontend::External},
+                             std::nullopt, true);
        ++It) {
     OptSpecifier Opt = [It, Matches]() {
       if (Matches(*It, frontend::Angled, true, true))
         return OPT_F;
       if (Matches(*It, frontend::Angled, false, true))
         return OPT_I;
+      if (Matches(*It, frontend::External, false, true))
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shafik wrote:

We should be striving to use [bugprone-argument-comment](https://clang.llvm.org/extra/clang-tidy/checks/bugprone/argument-comment.html) for literals like this. It looks like we do so below, it is unfortunate it is not applied consistently.

https://github.com/llvm/llvm-project/pull/105738