[CMake] Introduce LLVM_TARGET_TRIPLE_ENV as an option to override LLVM_DEFAULT_TARGET_TRIPLE at runtime.
No behavior is changed if LLVM_TARGET_TRIPLE_ENV is blank or undefined.
If LLVM_TARGET_TRIPLE_ENV is "TEST_TARGET_TRIPLE" and $TEST_TARGET_TRIPLE is not blank,
llvm::sys::getDefaultTargetTriple() returns $TEST_TARGET_TRIPLE.
Lit resets config.target_triple and config.environment[LLVM_TARGET_TRIPLE_ENV] to change the default target.
Without changing LLVM_DEFAULT_TARGET_TRIPLE nor rebuilding, lit can be run;
TEST_TARGET_TRIPLE=i686-pc-win32 bin/llvm-lit -sv path/to/test/
TEST_TARGET_TRIPLE=i686-pc-win32 ninja check-clang-tools
Differential Revision: https://reviews.llvm.org/D33662
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@305632
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