Take the following example:

    attribute mediump vec4 a; // location 0
    attribute highp   vec2 b; // location 0

    f(b)

This was previously translated to:

    attribute mediump vec4 a; // location 0

    f(a.xy)

That difference here is that b was previously highp, but a.xy is
mediump.  This change drops RelaxedPrecision from attributes so that
they are all highp.

Note that the temporary SPIR-V ids these attributes are loaded into
still retain their RelaxedPrecision decoration (if any), and the
precision of calculations in the shader is not increased by this
change.

Bug: angleproject:4249
Change-Id: Idca0fc667ad6e36ddf428b65ea03706272a9ed9c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2488131
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Charlie Lao <cclao@google.com>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>