AST: Avoid using SmallVector::set_size() in UnresolvedSet

Update UnresolvedSet to use (and expose) `SmallVector::truncate()` instead
of `SmallVector::set_size()`. The latter is going to made private in a
future commit to avoid misuse.

Differential Revision: https://reviews.llvm.org/D115386

clang/include/clang/AST/UnresolvedSet.h

@@ -121,7 +121,7 @@ public:
   void setAccess(iterator I, AccessSpecifier AS) { I.I->setAccess(AS); }
 
   void clear() { decls().clear(); }
-  void set_size(unsigned N) { decls().set_size(N); }
+  void truncate(unsigned N) { decls().truncate(N); }
 
   bool empty() const { return decls().empty(); }
   unsigned size() const { return decls().size(); }

clang/lib/Sema/SemaLookup.cpp

@@ -620,7 +620,7 @@ void LookupResult::resolveKind() {
     getSema().diagnoseEquivalentInternalLinkageDeclarations(
         getNameLoc(), HasNonFunction, EquivalentNonFunctions);
 
-  Decls.set_size(N);
+  Decls.truncate(N);
 
   if (HasNonFunction && (HasFunction || HasUnresolved))
     Ambiguous = true;