[ASTGEn] Handle _ identifier as the empty identifier

Author: DougGregor

lib/AST/CASTBridging.cpp

@@ -119,6 +119,9 @@ void SwiftDiagnostic_finish(BridgedDiagnostic diagPtr) {
 BridgedIdentifier
 SwiftASTContext_getIdentifier(void *ctx, const unsigned char *_Nullable str,
                               long len) {
+  if (len == 1 && str[0] == '_')
+    return BridgedIdentifier();
+
   // If this was a back-ticked identifier, drop the back-ticks.
   if (len >= 2 && str[0] == '`' && str[len-1] == '`') {
     ++str;

test/ASTGen/types.swift

@@ -31,3 +31,7 @@ func test12(_ producer: @escaping @autoclosure () -> Int) {
 func test12a(i: Int) {
   test12(i)
 }
+
+func test13(body: (_ value: Int) -> Void, i: Int) {
+  body(i)
+}