[WIP] Relax libcall-export-from-compartment warnings when the export is weak.

This is needed to unblock updating to a newer libc++ in CheriotRTOS.

Author: resistor

clang/lib/Sema/SemaDecl.cpp

@@ -15901,17 +15901,21 @@ Decl *Sema::ActOnStartOfFunctionDef(Scope *FnBodyScope, Decl *D,
       getCurLexicalContext()->getDeclKind() != Decl::ObjCImplementation)
     Diag(FD->getLocation(), diag::warn_function_def_in_objc_container);
 
-  if (FD->getType()->getAs<FunctionType>()->getCallConv() == CC_CHERILibCall &&
-      !Context.getLangOpts().CheriCompartmentName.empty())
-    Diag(FD->getLocation(),
-         diag::err_cheri_libcall_implemented_wrong_compartment)
-        << Context.getLangOpts().CheriCompartmentName;
-  else if (FD->hasAttr<CHERICompartmentNameAttr>() &&
-           (FD->getAttr<CHERICompartmentNameAttr>()->getCompartmentName() !=
-            Context.getLangOpts().CheriCompartmentName))
-    Diag(FD->getLocation(), diag::err_cheri_implemented_wrong_compartment)
-        << FD->getAttr<CHERICompartmentNameAttr>()->getCompartmentName()
-        << Context.getLangOpts().CheriCompartmentName;
+
+  auto GVA = Context.GetGVALinkageForFunction(FD);
+  if (GVA != GVA_AvailableExternally && GVA != GVA_DiscardableODR) {
+    if (FD->getType()->getAs<FunctionType>()->getCallConv() == CC_CHERILibCall &&
+        !Context.getLangOpts().CheriCompartmentName.empty())
+      Diag(FD->getLocation(),
+          diag::err_cheri_libcall_implemented_wrong_compartment)
+          << Context.getLangOpts().CheriCompartmentName;
+    else if (FD->hasAttr<CHERICompartmentNameAttr>() &&
+            (FD->getAttr<CHERICompartmentNameAttr>()->getCompartmentName() !=
+              Context.getLangOpts().CheriCompartmentName))
+      Diag(FD->getLocation(), diag::err_cheri_implemented_wrong_compartment)
+          << FD->getAttr<CHERICompartmentNameAttr>()->getCompartmentName()
+          << Context.getLangOpts().CheriCompartmentName;
+  }
 
   maybeAddDeclWithEffects(FD);