[clang] [concepts] Set up an instantiation scope for constraint expression comparison (PR #79698)

[Younan Zhang via cfe-commits]

zyn0217 wrote:

I've seen `llvm-libc++-shared.cfg.in :: libcxx/gdb/gdb_pretty_printer_test.sh.cpp` was failing intermittently on CI recently, which turned out to be unrelated to Concepts. In case I missed anything, I've run the libcxx tests locally, which are all green.

```txt
Testing Time: 1416.40s

Total Discovered Tests: 9636
  Unsupported      :  574 (5.96%)
  Passed           : 9037 (93.78%)
  Expectedly Failed:   25 (0.26%)
```

https://github.com/llvm/llvm-project/pull/79698