[llvm] Move AttributeMask to a separate header

Move `AttributeMask` out of `llvm/IR/Attributes.h` to a new file
`llvm/IR/AttributeMask.h`.  After doing this we can remove the
`#include <bitset>` and `#include <set>` directives from `Attributes.h`.
Since there are many headers including `Attributes.h`, but not needing
the definition of `AttributeMask`, this causes unnecessary bloating of
the translation units and slows down compilation.

This commit adds in the include directive for `llvm/IR/AttributeMask.h`
to the handful of source files that need to see the definition.

This reduces the total number of preprocessing tokens across the LLVM
source files in lib from (roughly) 1,917,509,187 to 1,902,982,273 - a
reduction of ~0.76%. This should result in a small improvement in
compilation time.

Differential Revision: https://reviews.llvm.org/D153728