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.