Add method to TargetInfo to get CPU cache line size

Summary:
This patch adds a virtual method `getCPUCacheLineSize()` to `TargetInfo`. Currently, I've only implemented the method in `X86TargetInfo`. It's extremely important that each CPU's cache line size correct (e.g., we can't just define it as `64` across the board) so, it has been a little slow getting to this point.

I'll work on the ARM CPUs next, but that will probably come later in a different patch.

Tags: #clang

Differential Revision: https://reviews.llvm.org/D74918