[clang] [clang][CrossTU] Indicate weak link attribute in ExternalDefMap. (PR #204188)

[Balázs Benics via cfe-commits]

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@@ -279,6 +306,8 @@ CrossTranslationUnitContext::~CrossTranslationUnitContext() {}
 std::optional<std::string>
 CrossTranslationUnitContext::getLookupName(const Decl *D) {
   SmallString<128> DeclUSR;
+  if (D->hasAttr<WeakAttr>())
+    DeclUSR.push_back('-');
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steakhal wrote:

This cannot be fixed at the CTU context level. it must be fixed within the `index` library, namely somewhere below `generateUSRForDecl`.

Please also explore if it would conflict with ObjectiveC interface declarations; some of those can have `-` and `+` signs in their declaration, which makes me worried about accidental mangling conflicts.

https://github.com/llvm/llvm-project/pull/204188