IRGen: address a TODO in the type metadata emission

Mark the explicit section for the nominal type metadata being emitted.  This is
constant data (post-relocation).  Map this to __TEXT,__const on MachO, .rodata
on ELF and .rdata on COFF.  Add the cases for COFF and ELF, and change the
switch to be covered.

Author: compnerd

lib/IRGen/GenMeta.cpp

@@ -1981,11 +1981,16 @@ llvm::Value *IRGenFunction::emitTypeLayoutRef(SILType type) {
 
 void IRGenModule::setTrueConstGlobal(llvm::GlobalVariable *var) {
   switch (TargetInfo.OutputObjectFormat) {
+  case llvm::Triple::UnknownObjectFormat:
+    llvm_unreachable("unknown object format");
   case llvm::Triple::MachO:
-    var->setSection("__TEXT, __const");
+    var->setSection("__TEXT,__const");
     break;
-  // TODO: ELF?
-  default:
+  case llvm::Triple::ELF:
+    var->setSection(".rodata");
+    break;
+  case llvm::Triple::COFF:
+    var->setSection(".rdata");
     break;
   }
 }

test/IRGen/nominal-type-section.swift

@@ -0,0 +1,11 @@
+// RUN: %swift -target x86_64-apple-macosx10.10 -emit-ir -parse-stdlib -primary-file %s -module-name main -o - | FileCheck %s -check-prefix CHECK-MACHO
+// RUN: %swift -target x86_64-unknown-linux-gnu -emit-ir -parse-stdlib -primary-file %s -module-name main -o - | FileCheck %s -check-prefix CHECK-ELF
+// RUN: %swift -target x86_64-unknown-windows-itanium -emit-ir -parse-stdlib -primary-file %s -module-name main -o - | FileCheck %s -check-prefix CHECK-COFF
+
+// CHECK-MACHO: @_TMnV4main1s = constant {{.*}}, section "__TEXT,__const"
+// CHECK-ELF: @_TMnV4main1s = {{.*}}constant {{.*}}, section ".rodata"
+// CHECK-COFF: @_TMnV4main1s = {{.*}}constant {{.*}}, section ".rdata"
+
+public struct s {
+}
+