IRGen: Fix asserting local extern declarations.

Explanation:
LLVM currently asserts on local extern variables in C headers passed to
Swift when the definition exists outside that header.

Scope: This never worked.

Risk: Low.

Testing: Regression test added.

Reviewed-by: John McCall

Original PR: swiftlang#33306

This fix also required the cherry-pick of: swiftlang#31035 and swiftlang#31272

rdar://67951491

Author: 3405691582

lib/IRGen/GenClangDecl.cpp

@@ -98,6 +98,11 @@ void IRGenModule::emitClangDecl(const clang::Decl *decl) {
         refFinder.TraverseDecl(executableDecl);
         next = executableDecl;
     }
+
+    if (auto var = dyn_cast<clang::VarDecl>(next))
+      if (!var->isFileVarDecl())
+        continue;
+
     ClangCodeGen->HandleTopLevelDecl(clang::DeclGroupRef(next));
   }
 }

test/IRGen/Inputs/local_extern.h

@@ -0,0 +1,21 @@
+static inline int _no_prior_var() {
+  extern int var;
+  return var;
+}
+
+static inline int _no_prior_func() {
+  extern int func();
+  return func();
+}
+
+static int prior_var = 1;
+static inline int _prior_var() {
+  extern int prior_var;
+  return prior_var;
+}
+
+static inline int prior_func() { return 1; }
+static inline int _prior_func() {
+  extern int prior_func();
+  return prior_func();
+}

test/IRGen/local_extern.swift

@@ -0,0 +1,10 @@
+// RUN: %target-swift-frontend -import-objc-header %S/Inputs/local_extern.h %s -emit-ir | %FileCheck %s
+// CHECK: @var = external {{(dso_local )?}}global i32
+// CHECK: @prior_var = internal global i32
+// CHECK: declare {{(dso_local )?}}i32 @func
+// CHECK: define internal i32 @prior_func
+
+print("\(_no_prior_var())")
+print("\(_no_prior_func())")
+print("\(_prior_var())")
+print("\(_prior_func())")