swift-synthesize-interface: Infer target triple.

Infer the `-target` argument to `swift-synthesize-interface` to be the host
triple when unspecified instead of emitting an error.

Resolves rdar://156353450.

Author: tshortli

lib/DriverTool/swift_synthesize_interface_main.cpp

@@ -84,8 +84,7 @@ int swift_synthesize_interface_main(ArrayRef<const char *> Args,
   if (auto *A = ParsedArgs.getLastArg(OPT_target)) {
     Target = llvm::Triple(A->getValue());
   } else {
-    Diags.diagnose(SourceLoc(), diag::error_option_required, "-target");
-    return EXIT_FAILURE;
+    Target = llvm::Triple(llvm::sys::getDefaultTargetTriple());
   }
 
   std::string OutputFile;

test/SynthesizeInterfaceTool/synthesize-interface.swift

@@ -1,4 +1,5 @@
 // RUN: %target-swift-synthesize-interface -module-name m1 -I %S/Inputs -o - | %FileCheck %s
+// RUN: %swift-synthesize-interface -module-name m1 -I %S/Inputs -o - | %FileCheck %s
 
 // CHECK:     public struct MyStruct {
 // CHECK-DAG:     public init()

test/lit.cfg

@@ -755,6 +755,7 @@ config.substitutions.append( ('%swift-path', config.swift) )
 config.substitutions.append( ('%swift-plugin-server', config.swift_plugin_server) )
 config.substitutions.append( ('%swift-parse-test', config.swift_parse_test) )
 config.substitutions.append( ('%swift-scan-test', config.swift_scan_test) )
+config.substitutions.append( ('%swift-synthesize-interface', config.swift_synthesize_interface) )
 config.substitutions.append( ('%validate-json', f"{config.python} -m json.tool") )
 
 config.clang_include_dir = make_path(config.llvm_obj_root, 'include')