SILLocation: a big refactoring and reducing its size from 3 to 2 words

My goal was to reduce the size of SILLocation. It now contains only of a storage union, which is basically a pointer and a bitfield containing the Kind, StorageKind and flags. By far, most locations are only single pointers to an AST node. For the few cases where more data needs to be stored, this data is allocated separately: with the SILModule's bump pointer allocator.

While working on this, I couldn't resist to do a major refactoring to simplify the code:

* removed unused stuff
* The term "DebugLoc" was used for 3 completely different things:
    - for `struct SILLocation::DebugLoc` -> renamed it to `FilePosition`
    - for `hasDebugLoc()`/`getDebugSourceLoc()` -> renamed it to `hasASTNodeForDebugging()`/`getSourceLocForDebugging()`
    - for `class SILDebugLocation` -> kept it as it is (though, `SILScopedLocation` would be a better name, IMO)
* made SILLocation more "functional", i.e. replaced some setters with corresponding constructors
* replaced the hand-written bitfield `KindData` with C bitfields
* updated and improved comments

Author: eeckstein

include/swift/SIL/PrettyStackTrace.h

@@ -20,6 +20,8 @@
 
 #include "swift/SIL/SILLocation.h"
 #include "swift/SIL/SILNode.h"
+#include "llvm/ADT/SmallString.h"
+#include "llvm/ADT/Twine.h"
 #include "llvm/Support/PrettyStackTrace.h"
 
 namespace swift {