Enable emission of macro expansion buffers for diagnostics by default

This feature is too handy to leave off-by-default. Enable it by
default, but allow `-emit-macro-expansion-files no-diagnostics` to
disable it.

Author: DougGregor

include/swift/Basic/DiagnosticOptions.h

@@ -74,7 +74,7 @@ class DiagnosticOptions {
   FormattingStyle PrintedFormattingStyle = FormattingStyle::LLVM;
 
   /// Whether to emit macro expansion buffers into separate, temporary files.
-  bool EmitMacroExpansionFiles = false;
+  bool EmitMacroExpansionFiles = true;
 
   std::string DiagnosticDocumentationPath = "";
 

test/Macros/macro_expand.swift

@@ -5,7 +5,7 @@
 // Diagnostics testing
 // RUN: %target-typecheck-verify-swift -enable-experimental-feature Macros -load-plugin-library %t/%target-library-name(MacroDefinition) -I %swift-host-lib-dir -module-name MacroUser -DTEST_DIAGNOSTICS
 
-// RUN: not %target-swift-frontend -typecheck -enable-experimental-feature Macros -load-plugin-library %t/%target-library-name(MacroDefinition) -I %swift-host-lib-dir -module-name MacroUser -DTEST_DIAGNOSTICS -serialize-diagnostics-path %t/macro_expand.dia %s
+// RUN: not %target-swift-frontend -typecheck -enable-experimental-feature Macros -load-plugin-library %t/%target-library-name(MacroDefinition) -I %swift-host-lib-dir -module-name MacroUser -DTEST_DIAGNOSTICS -serialize-diagnostics-path %t/macro_expand.dia %s -emit-macro-expansion-files no-diagnostics
 // RUN: c-index-test -read-diagnostics %t/macro_expand.dia 2>&1 | %FileCheck -check-prefix CHECK-DIAGS %s
 
 // Debug info SIL testing