Parse the attribute group reference on a function.

Parse the attribute group reference on a function.

Attribute references are of this form:

  define void @foo() #0 #1 #2 { ... }

Parse them for function attributes. If there's more than one reference, then
they are merged together.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174697 91177308-0d34-0410-b5e6-96231b3b80d8