Move the enum attributes defined in Attributes.h to a table-gen file.

This is a step towards consolidating some of the information regarding
attributes in a single place.

This patch moves the enum attributes in Attributes.h to the table-gen
file. Additionally, it adds definitions of target independent string
attributes that will be used in follow-up commits by the inliner to
check attribute compatibility.

rdar://problem/19836465

llvm-svn: 252796