[X86] Default to -mtune=generic unless -march is passed to the driver. Add...

[X86] Default to -mtune=generic unless -march is passed to the driver. Add TuneCPU to the AST serialization

This patch defaults to -mtune=generic unless -march is present. If -march is present we'll use the empty string unless its overridden by mtune. The back should use the target cpu if the tune-cpu isn't present.

It also adds AST serialization support to fix some tests that emit AST and parse it back. These tests diff the IR against the output from not going through AST. So if we don't serialize the tune CPU we fail the diff.

Differential Revision: https://reviews.llvm.org/D86488