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
gcc errors on this, but I'm nervous that since -mtune has been
ignored by clang for so long that there may be code bases out
there that pass 32-bit cpus to clang.