[Embedded] Only strip "external" from global variables that have definitions

Embedded Swift ends up rewriting the linkage of global variables as
part of linking together all of the Swift modules. Doing so for extern
global variables (e.g., ones meant to be implemented in C) breaks the
LLVM module, because they will never have definitions. Only make this
change when the global variable is a definition.

lib/SIL/IR/Linker.cpp

@@ -522,6 +522,7 @@ void SILLinkerVisitor::visitGlobalAddrInst(GlobalAddrInst *GAI) {
   // In Embedded Swift, we want to actually link globals from other modules too,
   // so strip "external" from the linkage.
   SILGlobalVariable *G = GAI->getReferencedGlobal();
+  if (G->isDefinition())
     G->setLinkage(stripExternalFromLinkage(G->getLinkage()));
 }
 

test/embedded/modules-extern.swift

@@ -21,8 +21,12 @@ public func publicFuncInAModule() {
 @usableFromInline
 internal func internalFuncInAModule() {
   some_c_api()
+  _ = globalVariable
 }
 
+@_extern(c)
+var globalVariable: Int
+
 // BEGIN Main.swift
 
 import MyModule