[clang] [Clang][Sema] Print more static_assert exprs (PR #74852)

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@@ -0,0 +1,117 @@
+// RUN: %clang_cc1 -std=c++2a -verify %s
+
+struct A {
+  int a, b[3], c;
+  bool operator==(const A&) const = default;
+};
+
+constexpr auto a0 = A{0, 0, 3, 4, 5};
+
+// expected-note at +1 {{evaluates to '{0, {0, 3, 4}, 5} == {1, {2, 3, 4}, 5}'}}
+static_assert(a0 == A{1, {2, 3, 4}, 5}); // expected-error {{failed}}
+
+struct _arr {
+  const int b[3];
+  constexpr bool operator==(const int rhs[3]) const {
+    for (unsigned i = 0; i < sizeof(b) / sizeof(int); i++)
+      if (b[i] != rhs[i])
+        return false;
+    return true;
+  }
+};
+
+// TODO[seth] syntactically sort of valid but almost entirely unusuable
+// (it's an int *, not an int [3] )
+// constexpr int _[3] = {...}; would work, but that's not piecewise substitutable
+// maybe it's ok? I mean, not like we can do better really...
+constexpr auto _ = (int[3]){2, 3, 4};
+
+// output: '{{2, 3, 4}} == {0, 3, 4}'  (the `{{` breaks VerifyDiagnosticConsumer::ParseDirective)
----------------
sethp wrote:

It didn't seem to like it; when I tried 

```
// expected-note at +1 {{evaluates to '\{{2, 3, 4}\} == {0, 3, 4}'}}
```

here I got this output from `-verify`:

```
error: 'expected-error' diagnostics seen but not expected: 
  File /home/seth/Code/src/github.com/llvm/llvm-project/clang/test/SemaCXX/static-assert-diagnostics.cpp Line 30: cannot find end ('}}') of expected string
error: 'expected-note' diagnostics seen but not expected: 
  File /home/seth/Code/src/github.com/llvm/llvm-project/clang/test/SemaCXX/static-assert-diagnostics.cpp Line 31: expression evaluates to '{{2, 3, 4}} == {0, 3, 4}'
2 errors generated.
```

I *think* I could use the `-re` mode, but then `{{` has _meaning_ so the output would be really obscured, something like `{{[{][{]}}` would be how to match `{{`, I think.

My bad idea is to teach the parser to count opening braces instead, so a `expected-error {{{` looks for a `}}}` and ignores any `{{` or `}}` sequences, but I wanted to get this up before dealing with that particular sub-problem.

https://github.com/llvm/llvm-project/pull/74852