Add Base.Experimental.@min_optlevel to set a min-optlevel for a module

[NHDaly]

Adds the complement to Experimental.@optlevel, which allows setting a min_optlevel.

    Experimental.@min_optlevel n::Int

Set the minimum optimization level (equivalent to the `--min-optlevel` command line
argument) for code in the current module. Submodules inherit the setting of their
parent module.

Supported values are 0, 1, 2, and 3.

This sets a lower-bound for optimization level, such that the effective optimization
level, `o` is `@min_optlevel <= o <= @optlevel`, overriding the defaults that come
from the command line arguments: `--min-optlevel <= o <= -O`.

NOTE: This min optimization level will only be applied to LLVM functions that are compiled
from this module. If a function defined in this module is *inlined* into a function defined
outside the module, that function will not inherit this min optimization level. Consider
whether you need to introduce `@noinline` if you require users to observe your min optlevel.

This is useful for the case where you have a very optimization-sensitive part of your codebase, but you otherwise want to run with a low optimization level such as -O0 or -O1, either for tests or even in production.

[nickrobinson251]

What happens when @optlevel and @min_optlevel disagree? Is it undefined, or is there a defined answer worth documenting?
e.g. both are used in the same module

module AA
@optlevel 1
@min_optlevel 2
...
end

or in submodules

module AA
@optlevel 1
    module BB
    @min_optlevel 2
    ...
    end
...
end