Introduce a new PartialAlias response for AliasAnalysis. For most

AliasAnalysis consumers, PartialAlias will be treated as MayAlias.

For AliasAnalysis chaining, MayAlias says "procede to the next analysis".
PartialAlias will be used to indicate that the query should terminate,
even though it didn't reach MustAlias or NoAlias.

llvm-svn: 121507