tint: Make CreateScalar and ZeroValue members

This avoids the need to pass the `use_runtime_semantics_` flag at each
callsite.

Change-Id: I2cce3f147226e1295b5dfa0239beeacd519d5bb4
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/118641
Commit-Queue: James Price <jrprice@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>

src/tint/resolver/const_eval.h

@@ -18,6 +18,7 @@
 #include <stddef.h>
 #include <string>
 
+#include "src/tint/number.h"
 #include "src/tint/type/type.h"
 #include "src/tint/utils/result.h"
 #include "src/tint/utils/vector.h"
@@ -1093,6 +1094,17 @@ class ConstEval {
     /// Adds the given note message to the diagnostics
     void AddNote(const std::string& msg, const Source& source) const;
 
+    /// CreateScalar constructs and returns a constant::Scalar<T>.
+    /// @param source the source location
+    /// @param t the result type
+    /// @param v the scalar value
+    /// @return the constant value with the same type and value
+    template <typename T>
+    ConstEval::Result CreateScalar(const Source& source, const type::Type* t, T v);
+
+    /// ZeroValue returns a Constant for the zero-value of the type `type`.
+    const constant::Value* ZeroValue(const type::Type* type);
+
     /// Adds two Number<T>s
     /// @param source the source location
     /// @param a the lhs number