Vulkan: Fix layout substitution for struct varyings If the shader contains code such as the following: struct S { vec4 field; }; out S varStruct; The layout qualifier macro is defined as @@ LAYOUT-varStruct @@. However, the Vulkan backend was replacing @@ LAYOUT-field @@. Bug: angleproject:3220 Change-Id: Iae15003867e0bed2cc939159a6653429c7a431e0 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1571389 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>