Follow-ups to issues earlier in the stack (#48455)

Summary:
Pull Request resolved: #48455

This is a followup to earlier issues in the stack.

Two fixes here:
1. Make the paths more consistent esp. from the config, so everything is working out of react-native-github.
2. Just declare `__cpluscplus`, as we don't seem to care about the value in our code.

Changelog: [Internal]

Reviewed By: cipolleschi

Differential Revision: D67716883

fbshipit-source-id: 359c33210d6b66bb0d75724a177587a7d5f837b2

tools/api/public-api.conf

@@ -7,6 +7,7 @@
 ; @format
 ; @oncall react_native
 
+; All [include] and [exclude] globs are run from the react-native project root
 [include]
 packages/react-native/**/*.h
 
@@ -22,6 +23,6 @@ packages/react-native/Libraries/WebSocket/*
 packages/react-native/Libraries/Wrapper/Example/*
 
 [settings]
-output=ReactNativeCPP.api
+output=tools/api/ReactNativeCPP.api
 ; clang=$HOME/some/path/to/clang
 ; clang-format=$HOME/some/path/to/clang-format

tools/api/public-api.js

@@ -133,10 +133,17 @@ function trimCPPNoise(
   if (filetype === FileType.UNKNOWN) {
     return;
   }
+  // src: https://en.cppreference.com/w/cpp/preprocessor/replace#Predefined_macros.
+  //
+  // I think we should only be define this without a specific version. It affects the preprocessor beyond simply gating code.
+  // This means we're forever going to introduce massive changes to the outputted API file that aren't meaningful for capturing
+  // user introduced API changes. This will affect our revision control commit logs.
+  const CPP20 = '202002L';
+
   // Strip comments, runs the preprocessor and removes formatting noise. The downside of this approach is it can be extremely
   // noisy if the preprocessor is able to resolve imports. This isn't the case the majority of the time.
   let sourceFileContents = execSync(
-    `${clang} -E -P -D__cplusplus=201703L ${sourcePath} 2> /dev/null | ${clangFormat}`,
+    `${clang} -E -P -D__cplusplus=${CPP20} ${sourcePath} 2> /dev/null | ${clangFormat}`,
     {encoding: 'utf-8'},
   )
     .toString()
@@ -191,8 +198,8 @@ function main() {
     `📚 Indexing header files took ${(performance.now() - start).toFixed(0)}ms`,
   );
 
-  // Write the output to a file
-  const outputFile = path.join(__dirname, config.settings.output);
+  // Write the output to a file. We lean on revision control to track changes.
+  const outputFile = path.join(GLOB_PROJECT_ROOT, config.settings.output);
   try {
     fs.unlinkSync(outputFile);
     log.info(`🔥 ${outputFile} already exists, deleting...`);