[Tblgen][MCA] Add the ability to mark groups as LoadQueue and StoreQueue. NFCI

Before this patch, users were not allowed to optionally mark processor resource
groups as load/store queues. That is because tablegen class MemoryQueue was
originally declared as expecting a ProcResource template argument (instead of a
more generic ProcResourceKind).

That was an oversight, since the original intention from D54957 was to let user
mark any processor resource as either load/store queue.  This patch adds the
ability to use processor resource groups in MemoryQueue definitions. This is not
a user visible change.

Differential Revision: https://reviews.llvm.org/D66810

llvm-svn: 370091