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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186552 91177308-0d34-0410-b5e6-96231b3b80d8