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/+/1571389Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>