[clang] [LifetimeSafety] Revamp test suite using unittests (PR #149158)

[Utkarsh Saxena via cfe-commits]

usx95 wrote:

Should be fixed by changing the name of the field to `AST` instead of `TestAST`
```
error: declaration of ‘std::unique_ptr<clang::TestAST> clang::lifetimes::internal::{anonymous}::LifetimeTestRunner::TestAST’ [-fpermissive]
   std::unique_ptr<TestAST> TestAST;
                            ^~~~~~~
In file included from /home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-multistage-test/clang-ppc64le-multistage/llvm/clang/unittests/Analysis/LifetimeSafetyTest.cpp:12:
/home/buildbots/llvm-external-buildbots/workers/ppc64le-clang-multistage-test/clang-ppc64le-multistage/llvm/clang/include/clang/Testing/TestAST.h:72:7: error: changes meaning of ‘TestAST’ from ‘class clang::TestAST’ [-fpermissive]
 class TestAST {
       ^~~~~~~
```

https://github.com/llvm/llvm-project/pull/149158