libstdc++: Add lightweight replacement for std::numeric_limits (PR 92546)
Many uses of std::numeric_limits in C++17 and C++20 features only really
need the min(), max() and digits constants for integral types. By adding
__detail::__int_limits we can avoid including the whole <limits> header.
The <limits> header isn't especially large, but avoiding it still gives
small savings in compilation time and memory usage for the compiler.
There are also C++11 features that could benefit from this change (e.g.
<bits/hashtable_policy.h> and <bits/uniform_int_dist.h>) but I won't
change those until stage 1.
The implementation of __int_limits assumes two's complement integers,
which is true for all targets supported by GCC.