[wgsl-in] Redefinition incorrectly passes validation

[Herschel]

Naga allows reusing a name in the same scope:

fn foo() {}
fn foo() {}

[[block]]
struct Globals {
    bar: vec4<f32>;
};

[[group(0), binding(0)]]
var<uniform> a: Globals;
[[group(0), binding(1)]]
var<uniform> a: Globals;

This passes validation in Naga, but this is not allowed by the WGSL spec:

A declaration must not introduce a name when that identifier is already in scope with the same end scope as another instance of that name.

Tint rejects the code with this message:

d:\projects\naga\color.wgsl:2:1 error: redefinition of 'foo'
fn foo() {}
^^

d:\projects\naga\color.wgsl:1:1 note: previous definition is here
fn foo() {}
^^

I hit this because learn wgpu book uses main for both vertex and fragment entry points. Tint disagrees with this, so I was caught by surprise when my shader worked in wgpu desktop but not Chrome.

naga @ 5cf11ab

[Herschel]

Sorry, just noticed this is somewhat of a dupe of #1044 and #1454. However, those issues discuss local variable shadowing, but the issue also occurs at module scope.

[jimblandy]

I think this was fixed in dba6beb. If that's not the case, please re-open.