tint/spirv/writer: Disable GCC false-positive warning

One of those false-positive maybe-initialized warnings deep in a STL class.

Change-Id: I25c38c9b0f1e8083d99075d7941018c80886ab9a
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/149683
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Auto-Submit: Ben Clayton <bclayton@google.com>
Reviewed-by: James Price <jrprice@google.com>
diff --git a/src/tint/lang/spirv/writer/ast_printer/builder.cc b/src/tint/lang/spirv/writer/ast_printer/builder.cc
index 7b0afae..fc8b55e 100644
--- a/src/tint/lang/spirv/writer/ast_printer/builder.cc
+++ b/src/tint/lang/spirv/writer/ast_printer/builder.cc
@@ -1,4 +1,4 @@
-// Copyright 2020 The Tint Authors.  //
+// Copyright 2020 The Tint Authors.
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
@@ -1931,6 +1931,8 @@
     }
 
     // Create the result matrix from the added/subtracted column vectors
+    TINT_BEGIN_DISABLE_WARNING(MAYBE_UNINITIALIZED);  // GCC false-positive
+
     auto result_mat_id = result_op();
     ops.insert(ops.begin(), result_mat_id);
     ops.insert(ops.begin(), Operand(GenerateTypeIfNeeded(type)));
@@ -1938,6 +1940,8 @@
         return 0;
     }
 
+    TINT_END_DISABLE_WARNING(MAYBE_UNINITIALIZED);  // GCC false-positive
+
     return std::get<uint32_t>(result_mat_id);
 }