[llvm-lit] Environment variable not found in clang subproject with lit internal shell

[Harini0924]

Currently, there are multiple environment variables are not recognized within the clang subproject, resulting in test failures when using the lit internal shell.
Example of error message

RUN: DEFAULT_TRIPLE=`%t/clang --version | grep "Target:" | cut -d ' ' -f2`

This error indicates that the internal shell is misinterpreting environment variable assignments as commands, leading to these failures.

files with failure:

• clang/test/ClangScanDeps/pr61006.cppm
• clang/test/Driver/coverage.c
• clang/test/Driver/program-path-priority.c

1. CMAKE config:

cmake -G Ninja -DLLVM_ENABLE_PROJECTS="clang;llvm;clang-tools-extra;lld;lldb;bolt;mlir;libclc;polly" -DLLVM_ENABLE_RUNTIMES="compiler-rt;libc;libcxx;libunwind;libcxxabi" -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_ASSERTIONS=ON -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -B build -S .

2. LIT_USE_INTERNAL_SHELL=1 ninja check-clang