c++ constexpr breaks with Mac OS native clang

The native Clang (Apple clang version 12.0.0 (clang-1200.0.32.28)) on Macs (Mac OS 10.15.7 anyway), seems to break bindgen's support for c++ header files. c header files seem fine. In particular, it seems to break the constexpr keyword.

$ echo 'constexpr int test = 1;' > test.hpp

$ bindgen --version
bindgen 0.56.0

$ bindgen test.hpp 
test.hpp:1:1: error: unknown type name 'constexpr'
test.hpp:1:1: error: unknown type name 'constexpr', err: true
thread 'main' panicked at 'Unable to generate bindings: ()', /Users/ccouzens/.cargo/registry/src/github.com-1ecc6299db9ec823/bindgen-0.56.0/src/main.rs:54:36
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

$ PATH="/usr/local/opt/llvm/bin:$PATH" bindgen test.hpp 
/* automatically generated by rust-bindgen 0.56.0 */

pub const test: ::std::os::raw::c_int = 1;

$ clang --version
Apple clang version 12.0.0 (clang-1200.0.32.28)
Target: x86_64-apple-darwin19.6.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

$ PATH="/usr/local/opt/llvm/bin:$PATH" clang --version
clang version 11.0.0
Target: x86_64-apple-darwin19.6.0
Thread model: posix
InstalledDir: /usr/local/opt/llvm/bin

Clang 12 hasn't yet been released. So it's odd that it's the version Apple is shipping. And it's not reasonable for bindgen to support a prerelease version.

I wanted to raise an issue:

• To help other people seeing this problem.
• To help the bindgen team prepare for clang 12 (if that's possible and reasonable)
• So that I (or someone else) can upgrade documentation such as requirements and the main readme.

https://github.com/ccouzens/tesseract-sys/issues/4

Reproduce the failure with the Apple clang and bindgen commands shown, then compare it with the Homebrew LLVM invocation. Review the linked requirements page and main README to determine whether the result calls for compatibility work or documentation; done requires an agreed resolution for Apple clang 12 behavior.