[MSL] Attribute `[[clang::optnone]]` causes significant performance hit on M1 Macs.

After briefly discussing commit fb3defc with @billhollings we decided to open a new ticket for this issue.
We tracked down a significant performance hit with the introduction of the [[clang::optnone]] attribute in UE5 (according to one of my colleagues from 100 fps down to 5 fps).
We only experience this regression on M1 Macs and in combination with our option invariant_float_math, but we solved the issue by removing all occurrences of that attribute via a simple std::string::find/replace after cross-compilation.
We can't simply disable invariant floating-point math, i.e. fma intrinsics, because we constantly had Z-fighting issues without it.

Our solution works for now, but it would be desirable to introduce a new option to allow to enable/disable the output of such attributes globally in SPIRV-Cross.
We haven't identified any issues with other attributes, so we only strip this attribute with a hardcoded string search-replace.

Start by tracing SPIRV-Cross's MSL output handling for the [[clang::optnone]] attribute, considering how it interacts with invariant_float_math. The change is done when a global option can suppress this attribute without disabling invariant floating-point math; no specific source file or test is named in the issue.