[clang-doc] `constexpr` fields are not handled well

Record fields are serialized via `QualType` because `RecordDecl::fields()` returns `FieldDecl`s, which aren't `VarDecl`s. Thus we have less information about them than other variables to document. One problem is that we don't capture their full spelling. For instance, in the basic project, we have:

https://github.com/llvm/llvm-project/blob/8b596224258dae1fc671d70e4f2072e162cd4f59/clang-tools-extra/test/clang-doc/Inputs/basic-project/include/Calculator.h#L61

`static_val` is `constexpr`, but we get its type spelling via `TypePrinter` which doesn't capture the `constexpr`. It only allows us to output `const` because that is actually part of the type. See:

https://github.com/llvm/llvm-project/blob/8b596224258dae1fc671d70e4f2072e162cd4f59/clang-tools-extra/test/clang-doc/basic-project.test#L332

I think it'd be pretty useful to have the actual source spelling since `constexpr` has more implications than just `const`. We also don't currently support documenting the initialization expression. These are both limited by the fact that we use type information to get the spelling, so we need to get to the `VarDecl` somehow.

Start with clang-tools-extra/test/clang-doc/Inputs/basic-project/include/Calculator.h and clang-tools-extra/test/clang-doc/basic-project.test, then trace how clang-doc serializes record fields through QualType. Update the coverage so the constexpr spelling and initialization expression are represented, and run basic-project.test to verify the documented output.

Written by the indexing model from the issue text.