[nfc] rename or eliminate isPureMoveOnly APIs

I think from SIL's perspective, it should only worry about whether the
type is move-only. That includes MoveOnlyWrapped SILTypes and regular
types that cannot be copied.

Most of the code querying `SILType::isPureMoveOnly` is in SILGen, where
it's very likely that the original AST type is sitting around already.
In such cases, I think it's fine to ask the AST type if it is
noncopyable. The clarity of only asking the ASTType if it's noncopyable
is beneficial, I think.

lib/SILOptimizer/SILCombiner/SILCombinerMiscVisitors.cpp

@@ -379,7 +379,7 @@ public:
     // analysis assumes memory is deinitialized on all paths, which is not the
     // case for discarded values. Eventually copyable types may also be
     // discarded; to support that, we will leave a drop_deinit_addr in place.
-    if (ASI->getType().isPureMoveOnly()) {
+    if (ASI->getType().getASTType()->isNoncopyable()) {
       LegalUsers = false;
       return;
     }