[clang] Suppress errors from well-formed-testing type traits in SFINAE contexts (PR #135390)

[Aaron Puchert via cfe-commits]

aaronpuchert wrote:

The tests would produce without the change:
```
error: 'expected-error' diagnostics seen but not expected:
  File clang/test/SemaCXX/type-traits.cpp Line 2676: calling a private constructor of class 'AllPrivate'
  File clang/test/SemaCXX/type-traits.cpp Line 2833: 'operator=' is a private member of 'AllPrivate'
  File clang/test/SemaCXX/type-traits.cpp Line 3209: calling a private constructor of class 'AllPrivate'
  File clang/test/SemaCXX/type-traits.cpp Line 4107: 'operator==' is a private member of 'is_trivially_equality_comparable::NotTriviallyEqualityComparablePrivateComparison'
4 errors generated.
```
and
```
error: 'expected-error' diagnostics seen but not expected:
  File clang/test/SemaCXX/type-trait-common-type.cpp Line 36: calling a private constructor of class 'PrivateConstructor'
1 error generated.
```

https://github.com/llvm/llvm-project/pull/135390