[wgsl-in] Rework Load Rule handling and indirection.

Make the parser code more closely follow the spec's grammar around
`unary_expression`, `postfix_expression`, and `singular_expression`.

Change the handling of postfix expressions (indexing, member/component access,
and swizzling) to apply the indirection at the appropriate time, resulting in
code improvements on all output formats. For example, where we used to generate
the following MSL:

    metal::float4 _e13 = bar.matrix[3];
    float b = _e13.x;

we now generate, simply:

    float b = bar.matrix[3].x;

Propagate WGSL reference types correctly, so that parenthesizing expressions no
longer causes the Load Rule to be applied.

Together with #1332 (already landed), this is a replacement for #1312, and
unblocks #1352.

Fixes #1351.