[DirectX] Shader Flag Analysis for UseNativeLowPrecision

[bharadwajy]

Implement analysis to set the flag if command-line option -enable-16bit-types is specified and Shader Model is 6.2 or later
NOTE: If this is set, min-precision mode (UseMinPrecision) is not set.

(Edit)
This analysis can be integrated into the existing DXIL Shader Flags Analysis pass (DXILShaderFlags.h/.cpp).
Implementation: Set the flag UseNativeLowPrecision true if and only if all of the following conditions are satisfied:

1. The NativeHalfType Lang Option (LangOptions.def) is true
2. Shader Model is 6.2 or later.

Condition 1 is equivalent to checking if -enable-16bit-types is specified on the command-line in clang-dxc.

Additional notes regarding the UseNativeLowPrecision flag:

• The flag is set even if the shader does not contain or use 16-bit types: https://godbolt.org/z/3xh6Eb13d
• The flag is set even in the presence of min- types (such as min16float): https://godbolt.org/z/znc49aTnP

[asl]

@bharadwajy Will it be possible for you to create meaningful titles from the beginning for the next issues you'll open? Note that notifications are sent with those UseNativeLowPrecision and this is very confusing and requires double-checking for not being spam. Thanks!