Rename TARGET_TRIPLE to LLVM_TARGET_TRIPLE (#1437)

Required after this change done in LLVM upstream:

    commit 0c0f6cfb7b9fab0ee69ef67ed95c194698c9d794
    Author: Petr Hosek <[email protected]>

    [CMake] Rename TARGET_TRIPLE to LLVM_TARGET_TRIPLE

    This clarifies that this is an LLVM specific variable and avoids
    potential conflicts with other projects.

    Differential Revision: https://reviews.llvm.org/D119918

Can be observed as:
lit.py: /usr/lib/llvm-15/build/utils/lit/lit/llvm/config.py:304: fatal:
Could not turn '' into Itanium ABI triple

Author: Pavel Chupin

test/lit.site.cfg.py.in

@@ -11,7 +11,7 @@ config.llvm_shlib_dir = "@SHLIBDIR@"
 config.llvm_plugin_ext = "@LLVM_PLUGIN_EXT@"
 config.lit_tools_dir = "@LLVM_LIT_TOOLS_DIR@"
 config.host_triple = "@LLVM_HOST_TRIPLE@"
-config.target_triple = "@TARGET_TRIPLE@"
+config.target_triple = "@LLVM_TARGET_TRIPLE@"
 config.host_arch = "@HOST_ARCH@"
 config.python_executable = "@PYTHON_EXECUTABLE@"
 config.test_run_dir = "@CMAKE_CURRENT_BINARY_DIR@"