Replace llvm::DIBuilder::DisableDebugLocations() with two RAII interfaces

inspired by CodegenFunction::LexicalScope.
- NoLocation temporarily turns off debug locations altogether.
  This is useful for emitting instructions that should be
  counted towards the function prologue.
- BuiltinLocation temporarily switches to an artificial debug location
  that has a valid scope, but no line information. This is useful when
  emitting compiler-generated helper functions that have no source
  location associated with them.

llvm-svn: 186552