[CMake][Fuchsia] Include llvm-undname

This is useful when developing on Windows.

Differential Revision: https://reviews.llvm.org/D122021

Author: petrhosek

clang/cmake/caches/Fuchsia-stage2.cmake

@@ -287,6 +287,7 @@ set(LLVM_TOOLCHAIN_TOOLS
   llvm-size
   llvm-strip
   llvm-symbolizer
+  llvm-undname
   llvm-xray
   sancov
   scan-build-py