[IRGen] Use PrivateLinkage for @__Swift_AST global

`.swift_ast` section is now recognized as a "metadata" section by LLVM's
object emission. On WebAssembly, metadata symbols must not appear in the
object file symbol table. Therefore, change the linkage of the
`@__Swift_AST` global from InternalLinkage to PrivateLinkage to avoid
exposing the symbol.

swiftlang/llvm-project@555eaef

Author: kateinoigakukun

lib/IRGen/IRGen.cpp

@@ -1851,8 +1851,12 @@ void swift::createSwiftModuleObjectFile(SILModule &SILMod, StringRef Buffer,
       llvm::ConstantDataArray::getString(IGM.getLLVMContext(),
                                          Buffer, /*AddNull=*/false);
   auto &M = *IGM.getModule();
+  // Use PrivateLinkage here. The ".swift_ast" section is specially recognized
+  // by LLVM’s object emission and classified as a "metadata" section. On
+  // WebAssembly, metadata symbols must not appear in the object file symbol
+  // table, so using PrivateLinkage avoids exposing the symbol.
   auto *ASTSym = new llvm::GlobalVariable(M, Ty, /*constant*/ true,
-                                          llvm::GlobalVariable::InternalLinkage,
+                                          llvm::GlobalValue::PrivateLinkage,
                                           Data, "__Swift_AST");
 
   std::string Section;