[ir] Move ir to `tint::core:ir` namespace

Update the `tint::ir` namespace to the new `tint::core::ir` location.

Bug: tint:1718
Change-Id: Id64e070328732302b64f75e7c73449a12528e824
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/147020
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
diff --git a/src/tint/lang/core/ir/convert.h b/src/tint/lang/core/ir/convert.h
index aaf5849..e9bfa44 100644
--- a/src/tint/lang/core/ir/convert.h
+++ b/src/tint/lang/core/ir/convert.h
@@ -19,7 +19,7 @@
 #include "src/tint/lang/core/type/type.h"
 #include "src/tint/utils/rtti/castable.h"
 
-namespace tint::ir {
+namespace tint::core::ir {
 
 /// A value conversion instruction in the IR.
 class Convert : public Castable<Convert, Call> {
@@ -37,6 +37,6 @@
     std::string_view FriendlyName() override { return "convert"; }
 };
 
-}  // namespace tint::ir
+}  // namespace tint::core::ir
 
 #endif  // SRC_TINT_LANG_CORE_IR_CONVERT_H_