[DirectX] Fix build break caused by bitcode writer changes

commit: 6894734
added a new `writeDIFixedPointType` function. However DIFixedPointType
is not supported in DXIL so we need to add a fail fast case for this to
fix the build.

this change fixes a build break introduced by #129596

Author: farzonl

llvm/lib/Target/DirectX/DXILWriter/DXILBitcodeWriter.cpp

@@ -237,6 +237,11 @@ class DXILBitcodeWriter {
                          SmallVectorImpl<uint64_t> &Record, unsigned Abbrev);
   void writeDIBasicType(const DIBasicType *N, SmallVectorImpl<uint64_t> &Record,
                         unsigned Abbrev);
+  void writeDIFixedPointType(const DIFixedPointType *N,
+                             SmallVectorImpl<uint64_t> &Record,
+                             unsigned Abbrev) {
+    llvm_unreachable("DXIL cannot contain DIFixedPointType Nodes");
+  }
   void writeDIStringType(const DIStringType *N,
                          SmallVectorImpl<uint64_t> &Record, unsigned Abbrev) {
     llvm_unreachable("DXIL cannot contain DIStringType Nodes");