FreeBSD: Drop system libxml2 dep in tools

This drops the dependency on the system libxml2 from the tools shipped
in the toolchain bundle. libxml2 is disabled entirely for the LLVM/lld
build, which uses it for processing the Windows manifest files. LLDB is
changed to link against the static libxml2 built for the FoundationXML
library. This is technically incorrect since the runtime may be
cross-compiled for a different platform than what the toolchain is built
to run on, but build-script is already mixing toolchain and runtime
content in inconsistent ways, so this is no worse off than we are
already.

libxml2 is updated pretty frequently in inconsistent ways, resulting in
failures to launch due to version incompatibilities.

utils/build-presets.ini

@@ -1374,6 +1374,9 @@ xctest
 swift-install-components=autolink-driver;compiler;clang-resource-dir-symlink;libexec;stdlib;swift-remote-mirror;sdk-overlay;static-mirror-lib;toolchain-tools;license;sourcekit-inproc
 llvm-install-components=llvm-ar;llvm-ranlib;llvm-cov;llvm-profdata;llvm-objdump;llvm-objcopy;llvm-symbolizer;IndexStore;clang;clang-resource-headers;builtins;runtimes;clangd;libclang;lld;LTO;clang-features-file
 
+extra-llvm-cmake-options=
+  -DLLVM_ENABLE_LIBXML2=OFF
+
 install-foundation
 install-libdispatch
 install-llbuild

utils/build-script-impl

@@ -2126,6 +2126,33 @@ for host in "${ALL_HOSTS[@]}"; do
                     DOTEST_EXTRA="${DOTEST_EXTRA} -Xlinker -rpath -Xlinker ${FOUNDATION_BUILD_DIR}/lib"
                 fi
 
+                if [[ ! "${SKIP_BUILD_LIBXML2}" ]]; then
+                  # Note: This is wrong when cross-compiling. LLDB is part of
+                  # the toolchain while the built libxml2 is built for the
+                  # runtime environment for use in FoundationXML.
+                  # Build-Script does not separate the toolchain and runtime
+                  # build, so this is already broken.
+                  BASE_INSTALL_DIR="$(get_host_install_destdir ${host})"
+                  LIBXML2_HEADERS="${BASE_INSTALL_DIR}/usr/include/libxml2"
+                  LIBXML2_LIBRARY="${BASE_INSTALL_DIR}/usr/lib/libxml2.a"
+
+                  if [[ -z "${DRY_RUN}" && ! -d "${LIBXML2_HEADERS}" ]]; then
+                      echo "Error: '${LIBXML2_HEADERS}' does not exist" 1>&2
+                      exit 1
+                  fi
+
+                  if [[ -z "${DRY_RUN}" && ! -e "${LIBXML2_LIBRARY}" ]]; then
+                      echo "Error: '${LIBXML2_LIBRARY}' does not exist" 1>&2
+                      exit 1
+                  fi
+
+                  cmake_options+=(
+                    -DLIBXML2_INCLUDE_DIR:PATH="${LIBXML2_HEADERS}"
+                    -DLIBXML2_LIBRARY:PATH="${LIBXML2_LIBRARY}"
+                    -DLIBXML2_DEFINITIONS="-DLIBXML_STATIC"
+                  )
+                fi
+
                 # Watchpoint testing is currently disabled: see rdar://38566150.
                 LLDB_TEST_CATEGORIES="--skip-category=watchpoint"