Add error messages for Swift module map parser

Parser errors with large Swift module map files can be hard to diagnose.
Refactor the parser to return an llvm::Error so clearer diagnostics can
be passed to the user.

lib/FrontendTool/FrontendTool.cpp

@@ -1604,8 +1604,12 @@ static bool generateReproducer(CompilerInstance &Instance,
     }
     auto map = llvm::json::parse(mapProxy->getData());
     if (!map) {
-      diags.diagnose(SourceLoc(), diag::explicit_swift_module_map_corrupted,
-                     mapOpts);
+      llvm::handleAllErrors(
+          map.takeError(), [&diags, &mapOpts](const llvm::json::ParseError &E) {
+            diags.diagnose(SourceLoc(),
+                           diag::explicit_swift_module_map_corrupted, mapOpts,
+                           E.message());
+          });
       return true;
     }
     if (auto array = map->getAsArray()) {