Reword the definition of the noalias attribute. The intention is for

the noalias argument on function attributes be usable to model the
C99 restrict keyword on arguments, and to allow AliasAnalysis to
consider a noalias-attributed argument to be an "identified object".

To support this, refactor a new "based on" concept out of the current
pointer aliasing "associated" concept. This "based on" concept is very
similar to (though it is not identical with) the "based on" concept
in C99.

Also, reword the definition of NoAlias to more closely describe the
concept that the optimizer uses.

llvm-svn: 107495