Refactor TryEvaluateMacroToConstant to simplify and dedup code (#6820)

For integral and float types, `TryEvaluateMacroToConstant` now calls
`MapAPValueToConstant` to directly convert from an APValue, rather than
converting the `APValue` to an expression and importing it with
`MapConstant`.

`MapConstant` is still used, but only for string literals and nullptrs.
Since it's only used by `TryEvaluateMacroToConstant`, moved it to
`macros.cpp` and removed the code for other types of expressions.

toolchain/check/cpp/constant.cpp

@@ -9,7 +9,6 @@
 
 namespace Carbon::Check {
 
-// TODO: dedup with code in `MapConstant` and `TryEvaluateMacroToConstant`.
 auto MapAPValueToConstant(Context& context, SemIR::LocId loc_id,
                           const clang::APValue& ap_value, clang::QualType type)
     -> SemIR::ConstantId {