ReadOnlyGlobalVariables: don't convert external global vars to lets

Because we don't know if external variables will be written

Fixes a miscompile
rdar://109476745

SwiftCompilerSources/Sources/Optimizer/ModulePasses/ReadOnlyGlobalVariables.swift

@@ -35,7 +35,8 @@ let readOnlyGlobalVariablesPass = ModulePass(name: "read-only-global-variables")
   }
 
   for g in moduleContext.globalVariables {
-    if !g.isPossiblyUsedExternally,
+    if !g.isAvailableExternally,
+       !g.isPossiblyUsedExternally,
        !g.isLet,
        !writtenGlobals.contains(g) {
       g.setIsLet(to: true, moduleContext)

SwiftCompilerSources/Sources/SIL/GlobalVariable.swift

@@ -35,6 +35,14 @@ final public class GlobalVariable : CustomStringConvertible, HasShortDescription
     return bridged.isPossiblyUsedExternally()
   }
 
+  /// True, if the linkage of the global indicates that it has a definition outside the
+  /// current compilation unit.
+  ///
+  /// For example, `public_external` linkage.
+  public var isAvailableExternally: Bool {
+    return bridged.isAvailableExternally()
+  }
+
   public var staticInitValue: SingleValueInstruction? {
     bridged.getStaticInitializerValue().instruction as? SingleValueInstruction
   }