[flang] Fix crash when handling benign USE conflict

When the same name is used for distinct derived types in two
modules, and at least one of those modules also defines a generic
interface of the same name, name resolution crashes when both
modules are USE'd into the same scope.  The crash is due to
some pointers into the symbol table becoming invalid when
a symbol is replaced with a UseErrorDetails; set them to null.
Also allow for extending a UseErrorDetails in place rather
than emitting a spurious error message.

Fixes llvm#121718.

flang/lib/Semantics/resolve-names.cpp

@@ -3162,6 +3162,10 @@ ModuleVisitor::SymbolRename ModuleVisitor::AddUse(
 // Convert it to a UseError with this additional location.
 static bool ConvertToUseError(
     Symbol &symbol, const SourceName &location, const Scope &module) {
+  if (auto *ued{symbol.detailsIf<UseErrorDetails>()}) {
+    ued->add_occurrence(location, module);
+    return true;
+  }
   const auto *useDetails{symbol.detailsIf<UseDetails>()};
   if (!useDetails) {
     if (auto *genericDetails{symbol.detailsIf<GenericDetails>()}) {
@@ -3319,6 +3323,8 @@ void ModuleVisitor::DoAddUse(SourceName location, SourceName localName,
         combinedDerivedType = CreateLocalUseError();
       } else {
         ConvertToUseError(*localSymbol, location, *useModuleScope_);
+        localDerivedType = nullptr;
+        localGeneric = nullptr;
         combinedDerivedType = localSymbol;
       }
     }

flang/test/Semantics/bug121718.f90

@@ -0,0 +1,31 @@
+! RUN: %flang_fc1 2>&1 | FileCheck %s --allow-empty
+! CHECK-NOT: error
+! Regression test simplified from LLVM bug 121718.
+! Ensure no crash and no spurious error message.
+module m1
+  type foo
+    integer x
+  end type
+ contains
+  subroutine test
+    print *, foo(123)
+  end
+end
+module m2
+  interface foo
+    procedure f
+  end interface
+  type foo
+    real x
+  end type
+ contains
+  complex function f(x)
+    complex, intent(in) :: x
+    f = x
+  end
+end
+program main
+  use m1
+  use m2
+  call test
+end