EDIT (by @kelset): Please read the current status of things at this comment.

Description

Running the latest Xcode/React Native etc. Upgrading an existing project failed so I tested with a new RN project (npx react-native init TestApp --version 0.64.0 --template react-native-template-typescript). It will build and run under Xcode 12.4, but fails when attempting to build with Xcode 12.5 Beta 3 (12E5244e). It's unlikely to be relevant, but the app is targeting an iPhone 12 Simulator running iOS 14.5

React Native version:

Run react-native info in your terminal and copy the results here.

System:
    OS: macOS 11.3
    CPU: (4) x64 Intel(R) Core(TM) i5-7360U CPU @ 2.30GHz
    Memory: 224.71 MB / 8.00 GB
    Shell: 3.2.57 - /bin/bash
  Binaries:
    Node: 15.11.0 - /var/folders/5h/g6n2qyx9781c5lwqmms28gw80000gn/T/yarn--1615999118104-0.9813238427222761/node
    Yarn: 1.22.10 - /var/folders/5h/g6n2qyx9781c5lwqmms28gw80000gn/T/yarn--1615999118104-0.9813238427222761/yarn
    npm: 7.6.3 - /usr/local/bin/npm
    Watchman: 4.9.0 - /usr/local/bin/watchman
  Managers:
    CocoaPods: 1.10.1 - /usr/local/bin/pod
  SDKs:
    iOS SDK:
      Platforms: iOS 14.5, DriverKit 20.4, macOS 11.3, tvOS 14.5, watchOS 7.4
    Android SDK: Not Found
  IDEs:
    Android Studio: Not Found
    Xcode: 12.5/12E5244e - /usr/bin/xcodebuild
  Languages:
    Java: javac 14 - /usr/bin/javac
  npmPackages:
    @react-native-community/cli: Not Found
    react: 17.0.1 => 17.0.1 
    react-native: 0.64.0 => 0.64.0 
    react-native-macos: Not Found
  npmGlobalPackages:
    *react-native*: Not Found
✨  Done in 15.64s.

Steps To Reproduce

Provide a detailed list of steps that reproduce the issue.

1. Install the above tools, Xcode 12.5 Beta 3, RN 0.64.0 etc.
2. Create a new app (npx react-native init TestApp --version 0.64.0 --template react-native-template-typescript)
3. cd testApp
4. yarn install then cd iOS & pod install
5. cd ../ & yarn ios
6. This error also appears if you replace step 4 with launching Xcode.app and building the app that way

Expected Results

Describe what you expected to happen.

A successful build of the example app, running on the simulator.

Snack, code example, screenshot, or link to a repository:

Please provide a Snack (https://snack.expo.io/), a link to a repository on GitHub, or provide a minimal code example that reproduces the problem.
You may provide a screenshot of the application if you think it is relevant to your bug report.
Here are some tips for providing a minimal example: https://stackoverflow.com/help/mcve

The following build commands failed:
CompileC /Users/harrygwinnell/Library/Developer/Xcode/DerivedData/testApp-hgqgknryyegjzjbbjfnfzrcyijmv/Build/Intermediates.noindex/Pods.build/Debug-iphonesimulator/Flipper-Folly.build/Objects-normal/x86_64/DistributedMutex.o /Users/harrygwinnell/Desktop/Git.nosync/testApp/testApp/ios/Pods/Flipper-Folly/folly/synchronization/DistributedMutex.cpp normal x86_64 c++ com.apple.compilers.llvm.clang.1_0.compiler
(1 failure)

This is the error that comes from yarn ios. A similar error comes from Xcode.app, reporting an error building flipper-folly.

Full console output available on request, but it was too big for pastern.

Xcode.app output is below