IRGen: dont use autolink-extract for COFF targets

COFF supports the `.drectve` section for embedding linker directives.  LLVM has
long supported emitting this section.  With this move, ELF shall become the only
target needing the autolink-extract functionality.

Author: compnerd

lib/IRGen/IRGenModule.cpp

@@ -879,6 +879,7 @@ void IRGenModule::emitAutolinkInfo() {
   switch (TargetInfo.OutputObjectFormat) {
   case llvm::Triple::UnknownObjectFormat:
     llvm_unreachable("unknown object format");
+  case llvm::Triple::COFF:
   case llvm::Triple::MachO: {
     llvm::LLVMContext &ctx = Module.getContext();
 
@@ -897,7 +898,6 @@ void IRGenModule::emitAutolinkInfo() {
     }
     break;
   }
-  case llvm::Triple::COFF:
   case llvm::Triple::ELF: {
     // Merge the entries into null-separated string.
     llvm::SmallString<64> EntriesString;

test/IRGen/autolink-coff.swift

@@ -0,0 +1,17 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: %swift -target thumbv7--windows-gnu -parse-as-library -parse-stdlib -emit-module-path %t/module.swiftmodule -module-name module -module-link-name module %s
+// RUN: %swift -target thumbv7--windows-gnu -parse-as-library -parse-stdlib -module-name autolink -I %t -D MAIN_MODULE -emit-ir -o - %s | FileCheck %s -check-prefix CHECK-GNU-IR
+// RUN: %swift -target thumbv7--windows-gnu -parse-as-library -parse-stdlib -module-name autolink -I %t -D MAIN_MODULE -S -o - %s | FileCheck %s -check-prefix CHECK-GNU-ASM
+
+#if MAIN_MODULE
+import module
+#endif
+
+// CHECK-GNU-IR: !{{[0-9]+}} = !{i32 {{[0-9]+}}, !"Linker Options", [[NODE:![0-9]+]]}
+// CHECK-GNU-IR: [[NODE]] = !{[[LIST:![0-9]+]]}
+// CHECK-GNU-IR: [[LIST]] = !{!"-lmodule"}
+
+// CHECK-GNU-ASM: .section .drectve
+// CHECK-GNU-ASM: .ascii " -lmodule"
+