Sema: warn when POSITION[0] is used intead of SV_Position

[Keenuts]

Before SM4, POSITION was equivalent to today's SV_Position. However, as HLSL moved to system semantics (SV_), the support for those old semantics has been deprecated. Some like VFACE are now completely forbidden, but others like POSITION or COLOR have been handled differently by FXC and DXC.

Today, usage of POSITION in place of SV_Position is probably a mistake as POSITION is now considered to be a user-defined semantic.

This commit adds a warning when POSITION is used as an output semantic for a vertex shader, as most cases should be the result of a mistake. If this is expected, the warning can be disabled by using -Wno-dx9-deprecation.

Fixes #3742

[Keenuts]

@pow2clk @tex3d

[Keenuts]

@llvm-beanz ping? (or @tex3d?)

[Keenuts]

@llvm-beanz friendly ping