[AttributeSet] Overload AttributeSet::addAttribute to reduce compile

time.

The new overloaded function is used when an attribute is added to a
large number of slots of an AttributeSet (for example, to function
parameters). This is much faster than calling AttributeSet::addAttribute
once per slot, because AttributeSet::getImpl (which calls
FoldingSet::FIndNodeOrInsertPos) is called only once per function
instead of once per slot.

With this commit, clang compiles a file which used to take over 22
minutes in just 13 seconds.

rdar://problem/23581000

Differential Revision: http://reviews.llvm.org/D15085

llvm-svn: 254491