[clang] [clang-repl] Remove invalid TopLevelStmtDecl from TU on parse failure (PR #153945)

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devajithvs wrote:

This was caught in one of the `cling-tests` in https://github.com/root-project/root/pull/19242, when comparing the state before and after a failed include.

CC: @vgvassilev 

https://github.com/llvm/llvm-project/pull/153945