[clang][PGO] Apply artificial DebugLoc to llvm.instrprof.increment instructions (#90717)

Prior to this change the debug-location for the
`llvm.instrprof.increment` intrinsic was set to whatever the current
DIBuilder's current debug location was set to. This meant that for
switch-statements, a counter's location was set to the previous case's
debug-location, causing confusing stepping behaviour in debuggers. This
patch makes sure we attach a dummy debug-location for the increment
instructions.

rdar://123050737