Provide a way to specify inliner's attribute compatibility and merging.

Provide a way to specify inliner's attribute compatibility and merging.

This reapplies r252949. I've changed the type of FuncName to be
std::string instead of StringRef in emitFnAttrCompatCheck.

Original commit message for r252949:

Provide a way to specify inliner's attribute compatibility and merging
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@252990 91177308-0d34-0410-b5e6-96231b3b80d8