MSVC: Enable compilation of C files with clang

For that to work, I had to drop pre-VS2015 support in `rt/msvc.c`. LDC
requires VS2015+ in `ldc.eh_msvc` anyway.

The nullfunc trick led to problems in 2 scenarios:

* Linking with LLD. I had to make sure the MS lib containing the
  __acrt_iob_func preceeded druntime in the linker command line, as it
  apparently happily chose nullfunc otherwise.
* Compiling the C source with clang. Even when linking with the MS
  linker, I couldn't get it to use the real __acrt_iob_func.

The `/alternatename` directive is totally undocumented, so I'm not too
surprised.

Author: kinke

src/ldc/msvc.c

@@ -99,6 +99,9 @@ typedef BOOL WINAPI _TLSCB(HINSTANCE, DWORD, LPVOID);
 #pragma section(".CRT$XLB", long, read)
 
 #pragma data_seg(".CRT$XLB")
+#ifdef __clang__
+__attribute__((unused))
+#endif
 __declspec(allocate(".CRT$XLB")) static _TLSCB* _pfix_tls = &fix_tlsAlignment;
 
 #pragma data_seg(pop)

src/rt/msvc.c

@@ -34,7 +34,9 @@ FILE* __iob_func();                 // VS2013-
 int _set_output_format(int format); // VS2013-
 
 //extern const char* __acrt_iob_func;
+#ifndef LDC
 extern const char* _nullfunc = 0;
+#endif
 
 #if defined _M_IX86
     #define C_PREFIX "_"
@@ -47,12 +49,19 @@ extern const char* _nullfunc = 0;
 #define DECLARE_ALTERNATE_NAME(name, alternate_name)  \
     __pragma(comment(linker, "/alternatename:" C_PREFIX #name "=" C_PREFIX #alternate_name))
 
+#ifndef LDC
 DECLARE_ALTERNATE_NAME (__acrt_iob_func, _nullfunc);
 DECLARE_ALTERNATE_NAME (__iob_func, _nullfunc);
 DECLARE_ALTERNATE_NAME (_set_output_format, _nullfunc);
+#endif
 
 void init_msvc()
 {
+#ifdef LDC
+    stdin = __acrt_iob_func(0);
+    stdout = __acrt_iob_func(1);
+    stderr = __acrt_iob_func(2);
+#else
     if (&__acrt_iob_func != (void*) &_nullfunc)
     {
         stdin = __acrt_iob_func(0);
@@ -71,22 +80,33 @@ void init_msvc()
         const int _TWO_DIGIT_EXPONENT = 1;
         _set_output_format(_TWO_DIGIT_EXPONENT);
     }
+#endif // !LDC
 }
 
+#ifdef LDC
+
+// needed for POSIX names
+#pragma comment(lib, "oldnames.lib")
+// VS2015+: printf/scanf family defined inline in C headers
+#pragma comment(lib, "legacy_stdio_definitions.lib")
+
+#else // !LDC
+
 // VS2015+ provides C99-conformant (v)snprintf functions, so weakly
 // link to legacy _(v)snprintf (not C99-conformant!) for VS2013- only
+
 DECLARE_ALTERNATE_NAME (snprintf, _snprintf);
 DECLARE_ALTERNATE_NAME (vsnprintf, _vsnprintf);
 
-DECLARE_ALTERNATE_NAME (fileno, _fileno);
-
 // VS2013- implements these functions as macros, VS2015+ provides symbols
+
 DECLARE_ALTERNATE_NAME (_fputc_nolock, _msvc_fputc_nolock);
 DECLARE_ALTERNATE_NAME (_fgetc_nolock, _msvc_fgetc_nolock);
 DECLARE_ALTERNATE_NAME (rewind, _msvc_rewind);
 DECLARE_ALTERNATE_NAME (clearerr, _msvc_clearerr);
 DECLARE_ALTERNATE_NAME (feof, _msvc_feof);
 DECLARE_ALTERNATE_NAME (ferror, _msvc_ferror);
+DECLARE_ALTERNATE_NAME (fileno, _msvc_fileno);
 
 // VS2013- helper functions
 int _filbuf(FILE* fp);
@@ -151,6 +171,13 @@ int  _msvc_ferror(FILE* stream)
     return stream->_flag & _IOERR;
 }
 
+int  _msvc_fileno(FILE* stream)
+{
+    return stream->_file;
+}
+
+#endif // !LDC
+
 
 
 /**
@@ -180,10 +207,3 @@ DECLARE_ALTERNATE_NAME (fmodf,  _msvc_fmodf);
 DECLARE_ALTERNATE_NAME (modff,  _msvc_modff);
 
 #endif // _M_IX86
-
-// LDC
-#pragma comment(lib, "oldnames.lib") // needed for POSIX names
-#if _MSC_VER >= 1900 // VS2015+
-    // needed for some symbols that are inline functions in the C headers
-    #pragma comment(lib, "legacy_stdio_definitions.lib")
-#endif