Fix UnDecorateSymbolNameFunc from dbghelp

UnDecorateSymbolName must be loaded from dbghelp, before it was always null.
The arguments of UnDecorateSymbolNameFunc were different than the Windows documentation.
The function DecorateSymbolName should only accept ASCII following what specified by its documentation, the Unicode version is UnDecorateSymbolNameW.

See https://docs.microsoft.com/en-us/windows/win32/api/dbghelp/nf-dbghelp-undecoratesymbolname

src/core/sys/windows/dbghelp.d

@@ -37,7 +37,7 @@ extern(System)
     alias DWORD64      function(HANDLE hProcess, DWORD64 dwAddr) SymGetModuleBase64Func;
     alias BOOL         function(HANDLE hProcess, DWORD64 dwAddr, IMAGEHLP_MODULEA64 *ModuleInfo) SymGetModuleInfo64Func;
     alias BOOL         function(HANDLE hProcess, DWORD64 Address, DWORD64 *Displacement, IMAGEHLP_SYMBOLA64 *Symbol) SymGetSymFromAddr64Func;
-    alias DWORD        function(PCTSTR DecoratedName, PTSTR UnDecoratedName, DWORD UndecoratedLength, DWORD Flags) UnDecorateSymbolNameFunc;
+    alias DWORD        function(PCSTR DecoratedName, PSTR UnDecoratedName, DWORD UndecoratedLength, DWORD Flags) UnDecorateSymbolNameFunc;
     alias DWORD64      function(HANDLE hProcess, HANDLE hFile, PCSTR ImageName, PCSTR ModuleName, DWORD64 BaseOfDll, DWORD SizeOfDll) SymLoadModule64Func;
     alias BOOL         function(HANDLE HProcess, PTSTR SearchPath, DWORD SearchPathLength) SymGetSearchPathFunc;
     alias BOOL         function(HANDLE hProcess, DWORD64 Address) SymUnloadModule64Func;
@@ -81,6 +81,7 @@ struct DbgHelp
             sm_inst.SymGetModuleBase64       = cast(SymGetModuleBase64Func) GetProcAddress(sm_hndl,"SymGetModuleBase64");
             sm_inst.SymGetModuleInfo64       = cast(SymGetModuleInfo64Func) GetProcAddress(sm_hndl,"SymGetModuleInfo64");
             sm_inst.SymGetSymFromAddr64      = cast(SymGetSymFromAddr64Func) GetProcAddress(sm_hndl,"SymGetSymFromAddr64");
+            sm_inst.UnDecorateSymbolName     = cast(UnDecorateSymbolNameFunc) GetProcAddress(sm_hndl,"UnDecorateSymbolName");
             sm_inst.SymLoadModule64          = cast(SymLoadModule64Func) GetProcAddress(sm_hndl,"SymLoadModule64");
             sm_inst.SymGetSearchPath         = cast(SymGetSearchPathFunc) GetProcAddress(sm_hndl,"SymGetSearchPath");
             sm_inst.SymUnloadModule64        = cast(SymUnloadModule64Func) GetProcAddress(sm_hndl,"SymUnloadModule64");
@@ -89,8 +90,8 @@ struct DbgHelp
             assert( sm_inst.SymInitialize && sm_inst.SymCleanup && sm_inst.StackWalk64 && sm_inst.SymGetOptions &&
                     sm_inst.SymSetOptions && sm_inst.SymFunctionTableAccess64 && sm_inst.SymGetLineFromAddr64 &&
                     sm_inst.SymGetModuleBase64 && sm_inst.SymGetModuleInfo64 && sm_inst.SymGetSymFromAddr64 &&
-                    sm_inst.SymLoadModule64 && sm_inst.SymGetSearchPath && sm_inst.SymUnloadModule64 &&
-                    sm_inst.SymRegisterCallback64 && sm_inst.ImagehlpApiVersion);
+                    sm_inst.UnDecorateSymbolName && sm_inst.SymLoadModule64 && sm_inst.SymGetSearchPath &&
+                    sm_inst.SymUnloadModule64 && sm_inst.SymRegisterCallback64 && sm_inst.ImagehlpApiVersion);
 
             return &sm_inst;
         }