Support #[cxx_name = "operator+="], etc. (i.e. not just C identifiers). #6983

Job	Run time
WebAssembly	34s
pre_ci / pre_ci	4s
Clang Tidy	0s
Outdated	0s
Bazel on ${{ (((matrix.os == 'ubuntu') && 'Linux') \|\| ((matrix.os == 'macos') && 'macOS') \|\| ((matrix.os == 'windows') && 'Windows') \|\| '???') }}	0s
ESLint	0s
Reindeer	0s
Clippy	0s
Emscripten	2m 6s
Minimal versions	27s
Documentation	47s
Cargo on Windows (msvc)	1m 50s
Rust 1.82.0	1m 5s
Clang (no exceptions)	44s
Rust beta	52s
C++17 on Linux	1m 16s
C++14 on Windows	1m 58s
C++17 on macOS	1m 16s
Rust nightly	1m 8s
C++14 on Linux	1m 12s
C++17 on Windows	1m 54s
Rust stable	1m 3s
Clang	1m 19s
C++20 on macOS	1m 7s
C++14 on macOS	53s
Cargo on macOS	56s
C++20 on Linux	1m 11s
C++20 on Windows	1m 55s
Pedantic	1m 9s
	26m 46s

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