-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