[clang/DIVar] Emit the flag for params that have unmodified value

Emit the debug info flag that indicates that a parameter has unchanged
value throughout a function.

([5/13] Introduce the debug entry values.)

Co-authored-by: Ananth Sowda <asowda@cisco.com>
Co-authored-by: Nikola Prica <nikola.prica@rt-rk.com>
Co-authored-by: Ivan Baev <ibaev@cisco.com>

Differential Revision: https://reviews.llvm.org/D58035

llvm-svn: 364424