Extend @_silgen_name to apply to allocating initializers, too

This enables its use on enum and struct initializers.

Author: DougGregor

lib/SIL/IR/SILDeclRef.cpp

@@ -1201,6 +1201,15 @@ std::string SILDeclRef::mangle(ManglingKind MKind) const {
                                           SKind);
 
   case SILDeclRef::Kind::Allocator:
+    // As a special case, initializers can have manually mangled names.
+    // Use the SILGen name only for the original non-thunked, non-curried entry
+    // point.
+    if (auto NameA = getDecl()->getAttrs().getAttribute<SILGenNameAttr>()) {
+      if (!NameA->Name.empty() && !isThunk()) {
+        return NameA->Name.str();
+      }
+    }
+
     return mangler.mangleConstructorEntity(cast<ConstructorDecl>(getDecl()),
                                            /*allocating*/ true,
                                            SKind);

test/IRGen/asmname.swift

@@ -51,3 +51,17 @@ private  func PlainPrivate()  { }
 // CHECK: define hidden void @cdecl_internal
 // CHECK: define hidden swiftcc void @"$s7asmname13CDeclInternal
 // CHECK: define internal void @cdecl_private()
+
+// silgen_name on enum constructors
+public enum X {
+case left(Int64)
+case right(Int64)
+}
+
+extension X {
+// CHECK: define{{( dllexport)?}}{{( protected)?}} swiftcc { i64, i8 } @blah_X_constructor
+  @_silgen_name("blah_X_constructor")
+  public init(blah: Int64) {
+    self = .left(blah)
+  }
+}