[WHLSL] Allow non-struct types as the return type of a shader

[litherum]

Migrated from https://bugs.webkit.org/show_bug.cgi?id=189387:

At 2018-09-07T00:10:37Z, tdenney@apple.com wrote:
float4 foo(...) : SV_TARGET { ... } is sugar for struct R { float4 stuff : SV_TARGET; } R foo(...) { ... }

[litherum]

At 2018-09-22T09:47:00Z, mmaxfield@apple.com wrote:

*** This bug has been marked as a duplicate of bug 189134 ***