[cxx-interop][SwiftCompilerSources] Do not use SWIFT_IMPORT_UNSAFE with SILDebugLocation

`SWIFT_IMPORT_UNSAFE` is an escape hatch that can be used to make the Swift compiler ignore its usual safety heuristics for C++ types.

`SILDebugLocation` is better described as a self-contained Swift type.

Similar to 35263ef.

Author: egorzhdan

include/swift/SIL/SILLocation.h

@@ -766,7 +766,6 @@ class SILDebugLocation {
   bool isAutoGenerated() const { return location.isAutoGenerated(); }
   operator bool() const { return bool(location) && debugScope; }
 
-  SWIFT_IMPORT_UNSAFE
   SILDebugLocation getAutogeneratedLocation() const {
     SILDebugLocation autoGenLoc(RegularLocation::getAutoGeneratedLocation(), getScope());
     return autoGenLoc;
@@ -784,7 +783,7 @@ class SILDebugLocation {
   static SILDebugLocation getArtificialUnreachableLocation() {
     return SILDebugLocation(ArtificialUnreachableLocation(), nullptr);
   }
-};
+} SWIFT_SELF_CONTAINED;
 
 } // end swift namespace