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/Frontend/Rewrite/RewriteObjC.cpp b/clang/lib/Frontend/Rewrite/RewriteObjC.cpp
index 37398e5..9938f89 100644
--- a/clang/lib/Frontend/Rewrite/RewriteObjC.cpp
+++ b/clang/lib/Frontend/Rewrite/RewriteObjC.cpp
@@ -1502,7 +1502,7 @@
   else {
     DeclRefExpr *DR = cast<DeclRefExpr>(S->getElement());
     elementName = DR->getDecl()->getName();
-    ValueDecl *VD = cast<ValueDecl>(DR->getDecl());
+    ValueDecl *VD = DR->getDecl();
     if (VD->getType()->isObjCQualifiedIdType() ||
         VD->getType()->isObjCQualifiedInterfaceType())
       // Simply use 'id' for all qualified types.