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/MicrosoftCXXABI.cpp b/clang/lib/AST/MicrosoftCXXABI.cpp
index b19491f..6d73716 100644
--- a/clang/lib/AST/MicrosoftCXXABI.cpp
+++ b/clang/lib/AST/MicrosoftCXXABI.cpp
@@ -106,7 +106,7 @@
   void addTypedefNameForUnnamedTagDecl(TagDecl *TD,
                                        TypedefNameDecl *DD) override {
     TD = TD->getCanonicalDecl();
-    DD = cast<TypedefNameDecl>(DD->getCanonicalDecl());
+    DD = DD->getCanonicalDecl();
     TypedefNameDecl *&I = UnnamedTagDeclToTypedefNameDecl[TD];
     if (!I)
       I = DD;