rules using table-gen. NFC.
This commit adds new classes CompatRule and MergeRule to Attributes.td,
which are used to generate code to check attribute compatibility and
merge attributes of the caller and callee.
rdar://problem/19836465
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@252949 91177308-0d34-0410-b5e6-96231b3b80d8