[clang-repl] Avoid use `$LD_LIBRARY_PATH` in lit tests (#208170)
Strengthen clang-repl lit tests by Avoid use `$LD_LIBRARY_PATH`. The
lit's internal shell does not expand shell variables such as
`$LD_LIBRARY_PATH`, so provide the current value as a lit substitution.
This PR can fix the following failures when user build llvm with
non-system C++ standard library.
```
# executed command: env 'LD_LIBRARY_PATH=<build dir>/tools/clang/test/Interpreter/Output/cxx20-modules.cppm.tmp:$LD_LIBRARY_PATH' <build dir>/bin/clang-repl -Xcc=-std=c++20 -Xcc=-fmodule-file=M=<build dir>/tools/clang/test/Interpreter/Output/cxx20-modules.cppm.tmp/mod.pcm -Xcc=--target=x86_64-linux-gnu
# .---command stderr------------
# | <build dir>/bin/clang-repl: /lib64/libstdc++.so.6: version `GLIBCXX_3.4.26' not found (required by <build dir>/bin/clang-repl)
# | <build dir>/bin/clang-repl: /lib64/libstdc++.so.6: version `GLIBCXX_3.4.29' not found (required by <build dir>/bin/clang-repl)
# | <build dir>/bin/clang-repl: /lib64/libstdc++.so.6: version `GLIBCXX_3.4.30' not found (required by <build dir>/bin/clang-repl)
# | <build dir>/bin/clang-repl: /lib64/libstdc++.so.6: version `CXXABI_1.3.13' not found (required by <build dir>/bin/clang-repl)
# `-----------------------------
```
Signed-off-by: yronglin <yronglin777@gmail.com>