Remove redundant casts. NFC

So I wrote a clang-tidy check to lint out redundant `isa`, `cast`, and
`dyn_cast`s for fun. This is a portion of what it found for clang; I
plan to do similar cleanups in LLVM and other subprojects when I find
time.

Because of the volume of changes, I explicitly avoided making any change
that wasn't highly local and obviously correct to me (e.g. we still have
a number of foo(cast<Bar>(baz)) that I didn't touch, since overloading
is a thing and the cast<Bar> did actually change the type -- just up the
class hierarchy).

I also tried to leave the types we were cast<>ing to somewhere nearby,
in cases where it wasn't locally obvious what we were dealing with
before.

llvm-svn: 326416
diff --git a/clang/lib/Sema/SemaObjCProperty.cpp b/clang/lib/Sema/SemaObjCProperty.cpp
index ea5b1da..3b8ad3d 100644
--- a/clang/lib/Sema/SemaObjCProperty.cpp
+++ b/clang/lib/Sema/SemaObjCProperty.cpp
@@ -618,7 +618,7 @@
     TInfo = Context.getTrivialTypeSourceInfo(T, TLoc);
   }
 
-  DeclContext *DC = cast<DeclContext>(CDecl);
+  DeclContext *DC = CDecl;
   ObjCPropertyDecl *PDecl = ObjCPropertyDecl::Create(Context, DC,
                                                      FD.D.getIdentifierLoc(),
                                                      PropertyId, AtLoc,