scudo: Replace a couple of macros with their expansions.

The macros INLINE and COMPILER_CHECK always expand to the same thing (inline
and static_assert respectively). Both expansions are standards compliant C++
and are used consistently in the rest of LLVM, so let's improve consistency
with the rest of LLVM by replacing them with the expansions.

Differential Revision: https://reviews.llvm.org/D70793