WinPIX: Find correct filename for subroutines in included headers within namespaces (microsoft#5776)

From a customer bug report, the previous code would skip over a
DISubProgram scope if that scope was contained in a DINamespace (as in
the first test case herein, wherein an include file is enclosed in a
namespace). This would result in DXIL being attributed to the includer
file instead of the included file.

Author: jeffnn

lib/DxilDia/DxcPixDxilDebugInfo.cpp

@@ -202,6 +202,8 @@ dxil_debug_info::DxcPixDxilSourceLocations::DxcPixDxilSourceLocations(
           S = ContainingScope;
         else
           S = S->getFile();
+      } else if (auto Subprogram = llvm::dyn_cast<llvm::DISubprogram>(S)) {
+        S = Subprogram->getFile();
       } else
         S = S->getScope().resolve(EmptyMap);
     }