tint/ast: Clean up StorageClass enum

Remove unused enum entries.
Add a ParseStorageClass() function. Have the WGSL parser use this.

First step to standardizing the way we parse enum-backed token sets.

Change-Id: I31c02816493beeabda740ff43946edce097f5fd1
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/97148
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>

src/tint/utils/string.h

@@ -15,6 +15,7 @@
 #ifndef SRC_TINT_UTILS_STRING_H_
 #define SRC_TINT_UTILS_STRING_H_
 
+#include <sstream>
 #include <string>
 
 namespace tint::utils {
@@ -34,6 +35,15 @@ inline std::string ReplaceAll(std::string str,
     return str;
 }
 
+/// @param value the value to be printed as a string
+/// @returns value printed as a string via the std::ostream `<<` operator
+template <typename T>
+std::string ToString(const T& value) {
+    std::stringstream s;
+    s << value;
+    return s.str();
+}
+
 }  // namespace tint::utils
 
 #endif  // SRC_TINT_UTILS_STRING_H_

src/tint/utils/string_test.cc

@@ -32,5 +32,10 @@ TEST(StringTest, ReplaceAll) {
     ASSERT_EQ("aabxybbxybcc", ReplaceAll("aabbcc", "b", "bxyb"));
 }
 
+TEST(StringTest, ToString) {
+    ASSERT_EQ("123", ToString(123));
+    ASSERT_EQ("hello", ToString("hello"));
+}
+
 }  // namespace
 }  // namespace tint::utils