clang emits useless guard variable for static const value

Please see https://godbolt.org/z/K6bM5qfx3 and compare the gcc and clang output

The compiler is able to calculate the `pow` results at compile time, however it still issues a __cxa_guard_acquire/__cxa_guard_release pair just to move those values around in memory. Meanwhile, gcc is able to use the .rodata section directly.

Start with the Godbolt reproducer at https://godbolt.org/z/K6bM5qfx3 and compare the Clang and GCC assembly output. Trace how Clang handles compile-time pow evaluation for the static const value and emits __cxa_guard_acquire/__cxa_guard_release. Done means the unnecessary guard pair is no longer emitted when the values can be placed directly in .rodata.

Written by the indexing model from the issue text.