Fix "pointer is null" static analyzer warning. NFCI.

Use castAs<> instead of getAs<> since the pointer is dereferenced immediately within mangleCallingConvention and castAs will perform the null assertion for us.
diff --git a/clang/lib/AST/MicrosoftMangle.cpp b/clang/lib/AST/MicrosoftMangle.cpp
index 91413ae..a286c53 100644
--- a/clang/lib/AST/MicrosoftMangle.cpp
+++ b/clang/lib/AST/MicrosoftMangle.cpp
@@ -710,7 +710,7 @@
   Out << "$B";
   mangleNumber(OffsetInVFTable);
   Out << 'A';
-  mangleCallingConvention(MD->getType()->getAs<FunctionProtoType>());
+  mangleCallingConvention(MD->getType()->castAs<FunctionProtoType>());
 }
 
 void MicrosoftCXXNameMangler::mangleName(const NamedDecl *ND) {