Initial submission for the attribute group feature.

Attribute groups are of the form:

  #0 = attributes { noinline "no-sse" "cpu"="cortex-a8" alignstack=4 }

Target-dependent attributes are represented as strings. Attributes can have
optional values associated with them. E.g., the "cpu" attribute has the value
"cortex-a8".

Target-independent attributes are listed as enums inside the attribute classes.

Multiple attribute groups can be referenced by the same object. In that case,
the attributes are merged together.

llvm-svn: 174493