Before this commit, the creation of (constant) attribute maps caused
`debug_info()` (which is built with `__attribute__((optnone))`) to
consume large amounts of stack space; up to tens of megabytes. This
caused problems particularly on macOS, where the default stack size
is 512 KiB.
After this commit, `std::map` objects are no longer created inline in
the `debug_info()` function, but are compiled to and then expanded from
a string literal in a subroutine call. This reduces stack space usage
by about 50%.
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