[llvm] Strip the full path from __FILE__ in the LDBG macro and keep only the filename (PR #150677)

[Nathan Chancellor via llvm-commits]

nathanchance wrote:

For what it's worth, I bisected my `CLANG_VENDOR` value appearing to disappear from the version string to this change.

```
$ cmake \
      -B build \
      -G Ninja \
      -S llvm \
      -Wno-dev \
      -DCLANG_ENABLE_STATIC_ANALYZER=OFF \
      -DCLANG_PLUGIN_SUPPORT=OFF \
      -DCLANG_VENDOR=ClangBuiltLinux \
      -DCMAKE_BUILD_TYPE=Release \
      -DCMAKE_CXX_COMPILER=clang++ \
      -DCMAKE_C_COMPILER=clang \
      -DLLVM_ENABLE_ASSERTIONS=ON \
      -DLLVM_ENABLE_BINDINGS=OFF \
      -DLLVM_ENABLE_LIBXML2=OFF \
      -DLLVM_ENABLE_OCAMLDOC=OFF \
      -DLLVM_ENABLE_PROJECTS=clang \
      -DLLVM_EXTERNAL_CLANG_TOOLS_EXTRA_SOURCE_DIR= \
      -DLLVM_INCLUDE_DOCS=OFF \
      -DLLVM_INCLUDE_EXAMPLES=OFF \
      -DLLVM_LINK_LLVM_DYLIB=OFF \
      -DLLVM_TARGETS_TO_BUILD=X86 \
      -DLLVM_USE_LINKER=lld
...

$ ninja -C build clang
ninja: Entering directory `build'
[2920/2920] Creating executable symlink bin/clang

$ build/bin/clang --version | head -1
clang version 22.0.0git (https://github.com/llvm/llvm-project.git 5d26e3c227f4b4a1761a8b0001b3165198def479)
```

At the parent change, there is no issue.

```diff
$ ninja -C build clang
ninja: Entering directory `build'
[2916/2916] Creating executable symlink bin/clang

$ build/bin/clang --version | head -1
ClangBuiltLinux clang version 22.0.0git (https://github.com/llvm/llvm-project.git 5ec6ac882c9fffaae6cf194f738e86f796394cd3)
```

https://github.com/llvm/llvm-project/pull/150677