Bake the _Concurrency -> Swift ABI module name into the compiler.

Using `-module-abi-name` for the `_Concurrency` module breaks older
Swift compilers. Instead, hard-code that "Swift" is the ABI name of the
"_Concurrency" module in the compiler to dodge the problem.

Author: DougGregor

include/swift/AST/Module.h

@@ -167,7 +167,7 @@ class ModuleDecl : public DeclContext, public TypeDecl {
   friend class DirectPrecedenceGroupLookupRequest;
 
   /// The ABI name of the module, if it differs from the module name.
-  Identifier ModuleABIName;
+  mutable Identifier ModuleABIName;
 
 public:
   /// Produces the components of a given module's full name in reverse order.
@@ -348,9 +348,7 @@ class ModuleDecl : public DeclContext, public TypeDecl {
 
   /// Retrieve the ABI name of the module, which is used for metadata and
   /// mangling.
-  Identifier getABIName() const {
-    return ModuleABIName.empty() ? getName() : ModuleABIName;
-  }
+  Identifier getABIName() const;
 
   /// Set the ABI name of the module;
   void setABIName(Identifier name) {

lib/AST/Module.cpp

@@ -1356,6 +1356,22 @@ ImportedModule::removeDuplicates(SmallVectorImpl<ImportedModule> &imports) {
   imports.erase(last, imports.end());
 }
 
+Identifier ModuleDecl::getABIName() const {
+  if (!ModuleABIName.empty())
+    return ModuleABIName;
+
+  // Hard code that the _Concurrency module has Swift as its ABI name.
+  // FIXME: This works around a backward-compatibility issue where
+  // -module-abi-name is not supported on existing Swift compilers. Remove
+  // this hack later and pass -module-abi-name when building the _Concurrency
+  // module.
+  if (getName().str() == SWIFT_CONCURRENCY_NAME) {
+    ModuleABIName = getASTContext().getIdentifier(STDLIB_NAME);
+    return ModuleABIName;
+  }
+
+  return getName();
+}
 
 StringRef ModuleDecl::getModuleFilename() const {
   // FIXME: Audit uses of this function and figure out how to migrate them to

stdlib/public/Concurrency/CMakeLists.txt

@@ -83,7 +83,6 @@ add_swift_target_library(swift_Concurrency ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} I
     ${SWIFT_STANDARD_LIBRARY_SWIFT_FLAGS}
     -parse-stdlib
     -Xfrontend -enable-experimental-concurrency
-    -module-abi-name Swift
   LINK_FLAGS "${SWIFT_RUNTIME_SWIFT_LINK_FLAGS}"
   INSTALL_IN_COMPONENT stdlib
 )

test/IRGen/module_abi_name.swift

@@ -2,14 +2,15 @@
 
 // CHECK-NOT: {{(5|")Hello}}
 
-// CHECK-DAG: [[MODULE_NAME:@.*]] = private constant [8 x i8] c"Goodbye\00"
-// CHECK-DAG: @"$s7GoodbyeMXM" = {{.*}} [[MODULE_NAME]]
-
-// CHECK-DAG: @"$s7Goodbye8GreetingCMm" =
+// CHECK: [[MODULE_NAME:@.*]] = private constant [8 x i8] c"Goodbye\00"
+// CHECK: @"$s7GoodbyeMXM" = {{.*}} [[MODULE_NAME]]
+// CHECK: @"$s7Goodbye8GreetingCMn" =
 public class Greeting { }
 
+
+
 // CHECK-NOT: {{(5|")Hello}}
-// CHECK: define swiftcc void @"$s7Goodbye8functionyyF"()
+// CHECK: define {{.*}} @"$s7Goodbye8functionyyF"
 @inlinable public func function() { }
 
 func callFunction() {