Add a few more c'tors:

* One that accepts a single Attribute::AttrKind.

* One that accepts an Attribute::AttrKind plus a list of values. This is for
  attributes defined like this:

    #1 = attributes { align = 4 }

* One that accepts a string, for target-specific attributes like this:

    #2 = attributes { "cpu=cortex-a8" }

llvm-svn: 171249