[libcxx] Call __count_bool_true for bitset count

This patch aims to help clang with better information so it can inline
__bit_reference count function usage for both std::biset. Current clang
inliner can not infer that the passed typed will be used only to select
the optimized variant, it evaluates the type argument and type check as
a load plus compare (although later optimization phases correctly
optimized this out).

It is mainly to help llvm inliner to generate better code for std::bitset
count for aarch64. It helps on both runtime and code size, since if inline
decides that _VSTD::count should not be inlined the vectorization will
create both aligned and unaligned variants (which add both code size and
runtime costs)

llvm-svn: 350936