Properly translate boolean statics to be stored as i8

While booleans are represented as i1 in SSA values, LLVM expects them
to be stored/loaded as i8 values. Using i1 as we do now works, but
kills some optimizations, so we should switch to i8, just like we do
everywhere else.

Fixes #16959.
auto merge of #17875 : dotdash/rust/static_bool, r=alexcrichton

While booleans are represented as i1 in SSA values, LLVM expects them
to be stored/loaded as i8 values. Using i1 as we do now works, but
kills some optimizations, so we should switch to i8, just like we do
everywhere else.

Fixes #16959.