[bazel] Remove unused dependency on libxml2

[aaronmondal]

Summary

The Bazel configs don't set LLVM_ENABLE_LIBXML2, so this was never usable to
begin with.

On systems without static libxml2.a this made lld runtime-dependent on an
unused, non-hermetic libxml2.so.

[GMNGeoffrey]

Hmm I'm trying to remember this history on this one too. I think this was added after hitting a real bug on Windows... Again @chandlerc

[aaronmondal]

In D143351#4107657, @GMNGeoffrey wrote:

Hmm I'm trying to remember this history on this one too. I think this was added after hitting a real bug on Windows... Again @chandlerc

This shouldn't affect anyone using default Bazel configs. The functionality that depends on libxml is disabled (as in, disabled on the preprocessor level). Not having xml2 support disables symbols in llvm/lib/WindowsManifest/WindowsManifestMerger.cpp, but linking xml2 it here without setting LLVM_ENABLE_XML2 has no effect.

This functionality should currently only be usable if you manually patched the Bazel configs.

Ultimately I'd like to make this a configurable flag depending on/setting LLVM_ENABLE_XML2, but I think the CMake-to-Bazel scripts are't ready to support this yet. It's probably better to wait for https://reviews.llvm.org/D143295 to get through first and then add configurability separately.

[GMNGeoffrey]

In D143351#4108160, @aaronmondal wrote:

This shouldn't affect anyone using default Bazel configs. The functionality that depends on libxml is disabled (as in, disabled on the preprocessor level). Not having xml2 support disables symbols in llvm/lib/WindowsManifest/WindowsManifestMerger.cpp, but linking xml2 it here without setting LLVM_ENABLE_XML2 has no effect.

This functionality should currently only be usable if you manually patched the Bazel configs.

Ultimately I'd like to make this a configurable flag depending on/setting LLVM_ENABLE_XML2, but I think the CMake-to-Bazel scripts are't ready to support this yet. It's probably better to wait for https://reviews.llvm.org/D143295 to get through first and then add configurability separately.

Yeah that makes sense. I'm just pretty sure that at the time there was some reason that Chandler added it to begin with and I'm trying to make sure we understand why it was there before removing it (Chesterton's Fence 🙂). Maybe it was indeed in service of a downstream who was patching the config. I think at Google we turn this on, but we use a hermetic version so this doesn't affect us either. Anyways, I'm pretty sure the current state is nonsensical as you say, so I'm ok to go ahead.

utils/bazel/llvm-project-overlay/llvm/BUILD.bazel

cc_library(
     name = "WindowsManifest",
     srcs = glob([
         "lib/WindowsManifest/*.cpp",
     ]),
     hdrs = glob([
         "include/llvm/WindowsManifest/*.h",
     ]),
     copts = llvm_copts,
-    linkopts = [
-        # Libxml2 is required to process Windows manifests. Without this,
-        # lld uses Microsoft mt.exe instead, which is not cross-platform.
-        "-lxml2",
-    ],
     deps = [
         ":Support",
         ":config",
     ],
 )
 
 cc_library(
     name = "MCA",