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/ItaniumMangle.cpp b/clang/lib/AST/ItaniumMangle.cpp
index 3c7e26d..d6d3792 100644
--- a/clang/lib/AST/ItaniumMangle.cpp
+++ b/clang/lib/AST/ItaniumMangle.cpp
@@ -1324,8 +1324,7 @@
 
     if (const VarDecl *VD = dyn_cast<VarDecl>(ND)) {
       // We must have an anonymous union or struct declaration.
-      const RecordDecl *RD =
-        cast<RecordDecl>(VD->getType()->getAs<RecordType>()->getDecl());
+      const RecordDecl *RD = VD->getType()->getAs<RecordType>()->getDecl();
 
       // Itanium C++ ABI 5.1.2:
       //