-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.
nit: Comments in the middle of the if make it a bit hard to read, and is not a common style in LLVM code. Please put the comments above the if describing the condition.