[HLSL] Support -E option for HLSL.

-E option will set entry function for hlsl.
The format is -E entry_name.

To avoid conflict with existing option with name 'E', add an extra prefix '--'.

A new field HLSLEntry is added to TargetOption.
To share code with HLSLShaderAttr, entry function will be add HLSLShaderAttr attribute too.

Reviewed By: beanz

Differential Revision: https://reviews.llvm.org/D124751