[glsl-in] naga rejects shader that uses lone vec4 expression.

[afd]

naga 74132ba rejects this valid shader:

#version 450

precision highp float;

precision highp int;

void main()
{
 vec4(1.0);
}

with:

error: Expected identifier or Semicolon, found LeftParen
  ┌─ 0013_reduced_final.frag:9:6
  │
9 │  vec4(1.0);
  │      ^

[NoelTautges]

This is due to a decision/TODO item in the parser to interpret statements starting with a type name or qualifier as declarations:

// TODO: This prevents snippets like the following from working
// ```glsl
// vec4(1.0);
// ```
// But this would require us to add lookahead to also support
// declarations and since this statement is very unlikely and most
// likely an error, for now we don't support it