[clang] [clang] Implement constexpr bit_cast for vectors (PR #66894)

[Richard Smith via cfe-commits]

================
@@ -7304,6 +7382,21 @@ class BufferToAPValueConverter {
     return ArrayValue;
   }
 
+  std::optional<APValue> visit(const VectorType *Ty, CharUnits Offset) {
+    SmallVector<uint8_t, 8> Bytes;
+    if (!Buffer.readObject(Offset, Info.Ctx.getTypeSizeInChars(Ty), Bytes))
+      return std::nullopt;
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zygoloid wrote:

... and here we'll need to not reject if the padding bits are uninitialized.

As a testcase, we should ensure that (for example) this:
```c++
struct A {
  long double a, b, c, d;
};
```
can round-trip through a vector of 4 `long double`s.

https://github.com/llvm/llvm-project/pull/66894