Add LLVM coverage conversion to LCOV format

Summary:
Changelog: [Internal]

This diff adds the ability to convert LLVM coverage JSON output to LCOV format for C++ code coverage in react-native-fantom.

Key additions:
- `convertLLVMCoverage.js`: Converts LLVM coverage data (segments, branches, functions) to LCOV format lines
- `types.flow.js`: Flow type definitions for LLVM coverage data structures
- Test utilities and fixtures for validating the conversion with real coverage data from various C++ files (AppSettings.cpp, Class.h, DevSettingsModule.h, NativeFantom.cpp/h, RawPropsKey.cpp)

This enables reporting C++ code coverage from Fantom tests in a standard format that can be consumed by coverage tools.

Differential Revision: D90135000

Author: andrewdacenko

private/react-native-fantom/runner/coverage/__test_utils__/loadLLVMCoverage.js

@@ -0,0 +1,96 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow strict-local
+ * @format
+ */
+
+import type {LLVMFileData, LLVMFunctionData} from '../convertLLVMCoverage';
+
+export type LLVMCoverage = {
+  file: LLVMFileData,
+  functions: Array<LLVMFunctionData>,
+  lcov: string[],
+};
+
+export default function loadLLVMCoverage(
+  name:
+    | 'AppSettings.cpp'
+    | 'Class.h'
+    | 'DevSettingsModule.h'
+    | 'NativeFantom.h'
+    | 'NativeFantom.cpp'
+    | 'RawPropsKey.cpp',
+): LLVMCoverage {
+  match (name) {
+    'AppSettings.cpp' => {
+      return {
+        // $FlowExpectedError[untyped-import]
+        file: require('./resources/AppSettings.cpp.json') as LLVMFileData,
+        functions:
+          // $FlowExpectedError[untyped-import]
+          require('./resources/AppSettings.cpp.functions.json') as Array<LLVMFunctionData>,
+        // $FlowExpectedError[untyped-import]
+        lcov: require('./resources/AppSettings.cpp.lcov.json') as string[],
+      };
+    }
+    'Class.h' => {
+      return {
+        // $FlowExpectedError[untyped-import]
+        file: require('./resources/Class.h.json') as LLVMFileData,
+        functions:
+          // $FlowExpectedError[untyped-import]
+          require('./resources/Class.h.functions.json') as Array<LLVMFunctionData>,
+        // $FlowExpectedError[untyped-import]
+        lcov: require('./resources/Class.h.lcov.json') as string[],
+      };
+    }
+    'DevSettingsModule.h' => {
+      return {
+        // $FlowExpectedError[untyped-import]
+        file: require('./resources/DevSettingsModule.h.json') as LLVMFileData,
+        functions:
+          // $FlowExpectedError[untyped-import]
+          require('./resources/DevSettingsModule.h.functions.json') as Array<LLVMFunctionData>,
+        // $FlowExpectedError[untyped-import]
+        lcov: require('./resources/DevSettingsModule.h.lcov.json') as string[],
+      };
+    }
+    'NativeFantom.cpp' => {
+      return {
+        // $FlowExpectedError[untyped-import]
+        file: require('./resources/NativeFantom.cpp.json') as LLVMFileData,
+        functions:
+          // $FlowExpectedError[untyped-import]
+          require('./resources/NativeFantom.cpp.functions.json') as Array<LLVMFunctionData>,
+        // $FlowExpectedError[untyped-import]
+        lcov: require('./resources/NativeFantom.cpp.lcov.json') as string[],
+      };
+    }
+    'NativeFantom.h' => {
+      return {
+        // $FlowExpectedError[untyped-import]
+        file: require('./resources/NativeFantom.h.json') as LLVMFileData,
+        functions:
+          // $FlowExpectedError[untyped-import]
+          require('./resources/NativeFantom.h.functions.json') as Array<LLVMFunctionData>,
+        // $FlowExpectedError[untyped-import]
+        lcov: require('./resources/NativeFantom.h.lcov.json') as string[],
+      };
+    }
+    'RawPropsKey.cpp' => {
+      return {
+        // $FlowExpectedError[untyped-import]
+        file: require('./resources/RawPropsKey.cpp.json') as LLVMFileData,
+        functions:
+          // $FlowExpectedError[untyped-import]
+          require('./resources/RawPropsKey.cpp.functions.json') as Array<LLVMFunctionData>,
+        // $FlowExpectedError[untyped-import]
+        lcov: require('./resources/RawPropsKey.cpp.lcov.json') as string[],
+      };
+    }
+  }
+}