[libcxx] [test] Use dedicated types for the invocable concept tests for multiple overloads

This should be clearer, instead of relying on rules for implicit
conversions regarding built in float/integer types.

Differential Revision: https://reviews.llvm.org/D99928

GitOrigin-RevId: 91d6debbb9136a0ac30278135bd4089c9e614ad1
2 files changed