Restrict JSON AST dumps to when -dump-ast is used.

Specifically, this means `-dump-ast-format json` is
incompatible with `-dump-parse`. This is because the JSON
format is meant to export more details about the AST that
require type checking to have been performed.

I'm open to lifting this restriction in the future.

Author: allevato

include/swift/AST/DiagnosticsFrontend.def

@@ -592,5 +592,8 @@ ERROR(experimental_not_supported_in_production,none,
 ERROR(zlib_not_supported,none,
        "this compiler was not built with zlib compression support enabled", ())
 
+ERROR(ast_format_requires_dump_ast,none,
+      "structured AST formats are only supported when using -dump-ast", ())
+
 #define UNDEFINE_DIAGNOSTIC_MACROS
 #include "DefineDiagnosticMacros.h"

lib/Frontend/ArgsToFrontendOptionsConverter.cpp

@@ -213,6 +213,11 @@ bool ArgsToFrontendOptionsConverter::convert(
             .Case("json", FrontendOptions::ASTFormat::JSON)
             .Case("json-zlib", FrontendOptions::ASTFormat::JSONZlib)
             .Default(FrontendOptions::ASTFormat::Default);
+    if (Opts.DumpASTFormat != FrontendOptions::ASTFormat::Default &&
+        !Args.hasArg(OPT_dump_ast)) {
+      Diags.diagnose(SourceLoc(), diag::ast_format_requires_dump_ast);
+      return true;
+    }
     if (Opts.DumpASTFormat == FrontendOptions::ASTFormat::JSONZlib &&
         !llvm::compression::zlib::isAvailable()) {
       Diags.diagnose(SourceLoc(), diag::zlib_not_supported);