Re-land [DebugInfo] Add debug location to stubs generated by CGDeclCXX and mark them as artificial

Previously, when clang was compiled with -DLLVM_ENABLE_ASSERTIONS=ON, the added tests were displaying:

inlinable function call in a function with debug info must have a !dbg location
  call void @"??1?$c@UB@@@@QEAA@XZ"(%struct.c* @"?f@?1??d@@YAPEAU?$c@UB@@@@XZ@4U2@A")
fatal error: error in backend: Broken module found, compilation aborted!
Stack dump:
0.      Program arguments: <f:\svn\buildninja\bin\clang -cc1 -emit-llvm debug-info-no-location.cpp> -gcodeview -debug-info-kind=limited
1.      <eof> parser at end of file
2.      Per-function optimization

Fixes PR43012

Differential Revision: https://reviews.llvm.org/D66328