Fix eh_win32_msvc.cc includes

[triplef]

When building on Windows using the latest Clang shipping with Visual Studio 2019 I’m getting the following errors:

../eh_win32_msvc.cc:65:13: error: no type named 'string' in namespace 'std'
static std::string mangleObjcObject()
       ~~~~~^
../eh_win32_msvc.cc:68:9: error: cannot initialize return object of type 'int' with an lvalue of type 'const char [19]'
        return ".PEAUobjc_object@@";
               ^~~~~~~~~~~~~~~~~~~~
../eh_win32_msvc.cc:74:13: error: no type named 'string' in namespace 'std'
static std::string mangleStructNamed(const char* className)
       ~~~~~^
../eh_win32_msvc.cc:83:8: error: no member named 'string' in namespace 'std'
                std::string(".PEAU") +
                ~~~~~^

Changing the "string.h" include to just "string" fixes the issues. I also updated the other two C includes to their C++ counterparts.

Not sure why it works in the CI, I’m guessing it must be a difference between the Visual Studio Clang build and the one from Chocolatey.