Lazily IR-declare global variables

Analogous to ldc-developers#4420.

Author: kinke

gen/declarations.cpp

@@ -236,9 +236,12 @@ class CodegenVisitor : public Visitor {
                            decl->toPrettyChars());
     LOG_SCOPE;
 
-    if (decl->ir->isDefined()) {
+    if (decl->ir->isDefined())
+      return;
+
+    // skip external declarations (IR-declared lazily)
+    if (decl->storage_class & STCextern)
       return;
-    }
 
     if (decl->type->ty == TY::Terror) {
       decl->error("had semantic errors when compiling");

tests/codegen/avr.d

@@ -11,3 +11,5 @@ version (D_SoftFloat) {} else static assert(0);
 int definedGlobal = 123;
 // CHECK: @_D3avr14declaredGlobali = external global i32
 extern int declaredGlobal;
+
+int dummyRef() { return declaredGlobal; } // make sure `declaredGlobal` is IR-declared

tests/codegen/wasi.d

@@ -15,8 +15,6 @@ version (CRuntime_WASI) {} else static assert(0);
 
 // CHECK: @_D4wasi13definedGlobali = global i32 123
 int definedGlobal = 123;
-// CHECK: @_D4wasi14declaredGlobali = external global i32
-extern int declaredGlobal;
 
 
 // make sure the ModuleInfo ref is emitted into the __minfo section:

tests/fail_compilation/global_var_collision.d

@@ -1,8 +1,17 @@
+// It should compile fine when not referencing the colliding external global:
+// RUN: %ldc -c %s -d-version=DontReference
+
+// But fail if referenced:
 // RUN: not %ldc -c %s 2>&1 | FileCheck %s
 
 extern(C) extern int myGlobal;
 
-// CHECK:      global_var_collision.d(9): Error: Global variable type does not match previous declaration with same mangled name: `myGlobal`
+version (DontReference) {} else
+{
+    int dummyRef() { return myGlobal; }
+}
+
+// CHECK:      global_var_collision.d([[@LINE+4]]): Error: Global variable type does not match previous declaration with same mangled name: `myGlobal`
 // CHECK-NEXT: Previous IR type: i32, mutable, thread-local
 // CHECK-NEXT: New IR type:      i64, const, non-thread-local
 pragma(mangle, myGlobal.mangleof)