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/AST/CXXInheritance.cpp b/clang/lib/AST/CXXInheritance.cpp
index 24e96ba..9314cfd 100644
--- a/clang/lib/AST/CXXInheritance.cpp
+++ b/clang/lib/AST/CXXInheritance.cpp
@@ -637,8 +637,7 @@
                                OMEnd = BaseOverriders->end();
            OM != OMEnd;
            ++OM) {
-        const CXXMethodDecl *CanonOM
-          = cast<CXXMethodDecl>(OM->first->getCanonicalDecl());
+        const CXXMethodDecl *CanonOM = OM->first->getCanonicalDecl();
         Overriders[CanonOM].add(OM->second);
       }
     }
@@ -649,7 +648,7 @@
     if (!M->isVirtual())
       continue;
 
-    CXXMethodDecl *CanonM = cast<CXXMethodDecl>(M->getCanonicalDecl());
+    CXXMethodDecl *CanonM = M->getCanonicalDecl();
     using OverriddenMethodsRange =
         llvm::iterator_range<CXXMethodDecl::method_iterator>;
     OverriddenMethodsRange OverriddenMethods = CanonM->overridden_methods();