tint: Rename kInvalid to kUndefined for enums

This is a more neutral term for an unassigned enum value.

Change-Id: Ic69d912472f26fd8a2c8348281b27edfcc145eab
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/105480
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>

src/tint/sem/builtin.cc

@@ -182,7 +182,7 @@ ast::Extension Builtin::RequiredExtension() const {
     if (IsDP4a()) {
         return ast::Extension::kChromiumExperimentalDp4A;
     }
-    return ast::Extension::kInvalid;
+    return ast::Extension::kUndefined;
 }
 
 }  // namespace tint::sem

src/tint/sem/pointer.cc

@@ -25,7 +25,7 @@ namespace tint::sem {
 Pointer::Pointer(const Type* subtype, ast::AddressSpace address_space, ast::Access access)
     : subtype_(subtype), address_space_(address_space), access_(access) {
     TINT_ASSERT(Semantic, !subtype->Is<Reference>());
-    TINT_ASSERT(Semantic, access != ast::Access::kInvalid);
+    TINT_ASSERT(Semantic, access != ast::Access::kUndefined);
 }
 
 size_t Pointer::Hash() const {

src/tint/sem/reference.cc

@@ -24,7 +24,7 @@ namespace tint::sem {
 Reference::Reference(const Type* subtype, ast::AddressSpace address_space, ast::Access access)
     : subtype_(subtype), address_space_(address_space), access_(access) {
     TINT_ASSERT(Semantic, !subtype->Is<Reference>());
-    TINT_ASSERT(Semantic, access != ast::Access::kInvalid);
+    TINT_ASSERT(Semantic, access != ast::Access::kUndefined);
 }
 
 size_t Reference::Hash() const {

src/tint/sem/storage_texture.cc

@@ -75,7 +75,7 @@ sem::Type* StorageTexture::SubtypeFor(ast::TexelFormat format, sem::TypeManager&
             return type_mgr.Get<sem::F32>();
         }
 
-        case ast::TexelFormat::kInvalid:
+        case ast::TexelFormat::kUndefined:
             break;
     }