[IRGen] Fix asserting local extern declarations.

emitClangDecl interacts with clang and LLVM to achieve C interop. On the
LLVM side, CodeGenModule::EmitGlobal asserts if the decl eventually
passed to it is not a "file scoped" via VarDecl::isFileVarDecl.

LLVM currently asserts on local extern variables in C headers passed to
Swift when the definition exists outside that header. To fix this, we
need to ensure that we are only passing Decls that do not trip the
assertion but not unduly limit local extern variables when the
corresponding definition exists inside that header.

We can do that fairly simply by checking for isFileVarDecl just before
we hand-off to clang. When the definition for the local extern variable
exists inside the header, isFileVarDecl is true, and if it exists
elsewhere, it is false. This matches up with the assert expectation on
the LLVM side exactly.

This indirectly addresses #28968, since that contains the only part of
the Swift stdlib that uses a local extern variable, but any header that
is used with Swift that contains a local extern variable will cause the
compiler to assert when built with assertions enabled.

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 global i32
+// CHECK: @prior_var = internal global i32
+// CHECK: declare i32 @func
+// CHECK: define internal i32 @prior_func
+
+print("\(_no_prior_var())")
+print("\(_no_prior_func())")
+print("\(_prior_var())")
+print("\(_prior_func())")