[clang] [Clang] Use valid source loc for empty target_clones diagnostic (PR #173747)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-x86_64-linux-abi-test` running on `sie-linux-worker2` while building `clang` at step 6 "build-unified-tree".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/8/builds/25177

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 6 (build-unified-tree) failure: build (failure)
...
1048.835 [1503/10/5973] Building CXX object tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/AST/ASTDumperTest.cpp.o
1061.606 [1502/10/5974] Building CXX object tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/AST/ASTExprTest.cpp.o
1064.005 [1501/10/5975] Building CXX object tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/ASTMatchers/ASTMatchersNodeTest.cpp.o
1065.958 [1500/10/5976] Building CXX object tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/AST/ASTImporterFixtures.cpp.o
1066.019 [1499/10/5977] Building CXX object tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/AST/ASTVectorTest.cpp.o
1068.694 [1498/10/5978] Building CXX object tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/AST/ASTImporterObjCTest.cpp.o
1068.755 [1497/10/5979] Building CXX object tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/AST/CommentLexer.cpp.o
1068.817 [1496/10/5980] Building CXX object tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/AST/CommentParser.cpp.o
1068.869 [1495/10/5981] Building CXX object tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/AST/CommentTextTest.cpp.o
1075.019 [1494/10/5982] Building CXX object tools/clang/unittests/CMakeFiles/AllClangUnitTests.dir/ASTMatchers/ASTMatchersNarrowingTest.cpp.o
command timed out: 1200 seconds without output running [b'ninja'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=2392.377988

```

</details>

https://github.com/llvm/llvm-project/pull/173747