[clang] Add missing header include

With my version of the MSVC tools (14.11.25503), this was failing to
build because of missing declarations of `std::isalnum` and
`std::isdigit`. Include `<cctype>` to get these.

Author: smeenai

clang/lib/Driver/ToolChains/Clang.cpp

@@ -52,6 +52,7 @@
 #include "llvm/Support/Process.h"
 #include "llvm/Support/TargetParser.h"
 #include "llvm/Support/YAMLParser.h"
+#include <cctype>
 
 using namespace clang::driver;
 using namespace clang::driver::tools;