[clad] Bump clad version to v0.3.

The new release includes some improvements in both Forward and
Reverse mode:
  * Better correctness of C++ constructs -- handle scopes properly; allow proper
  variable shadowing; and preserve namespaces.
  * Efficient evaluation in forward mode.
  * Reduced cloning complexity.
  * Handle more C++ constructs -- variable reassignments and for loops.

See more at: https://github.com/vgvassilev/clad/blob/v0.3/docs/ReleaseNotes.md

interpreter/cling/tools/plugins/clad/CMakeLists.txt

@@ -22,7 +22,7 @@ if(MSVC)
   ExternalProject_Add(
     clad
     GIT_REPOSITORY https://github.com/vgvassilev/clad.git
-    GIT_TAG v0.2
+    GIT_TAG v0.3
     UPDATE_COMMAND ""
     CMAKE_ARGS -G ${CMAKE_GENERATOR} -DCLAD_BUILD_STATIC_ONLY=ON
                -DCMAKE_INSTALL_PREFIX=${CLING_PLUGIN_INSTALL_PREFIX}