BitVector.h:capacity_in_bytes Don't mark header functions as file-scope static

This creates ODR violations if the function is called from another
inline function in a header and also creates binary bloat from duplicate
definitions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316470 91177308-0d34-0410-b5e6-96231b3b80d8