MSVC 2017 build fixes

Add missing include, and fix return type of tint::utils::UniqueVector::empy().

Fixed: tint:1395
Change-Id: I413eabb882b39d959dedcc4175cd7f5cc82e6838
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/77840
Auto-Submit: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>

src/reader/wgsl/lexer.cc

@@ -14,6 +14,7 @@
 
 #include "src/reader/wgsl/lexer.h"
 
+#include <cctype>
 #include <cmath>
 #include <cstring>
 #include <limits>

src/utils/unique_vector.h

@@ -73,7 +73,7 @@ struct UniqueVector {
   const T& operator[](size_t i) const { return vector[i]; }
 
   /// @returns true if the vector is empty
-  size_t empty() const { return vector.empty(); }
+  bool empty() const { return vector.empty(); }
 
   /// @returns the number of items in the vector
   size_t size() const { return vector.size(); }