clang: Add a cmake toggle to disable libclang's libxml2 dependency

[thakis]

Summary

By default, libclang will still depend on libxml2 according to
LLVM_ENABLE_LIBXML2, but with this it's possible to make libclang
not depend on libxml2 even though LLVM does.

The motivation is that libclang uses a bigger API surface of libxml2
than LLVM, and we're statically linking a very minimal build of
libxml2 in our LLVM/clang distribution.

[hans]

lgtm

clang/CMakeLists.txt

 
 # Make sure that our source directory is on the current cmake module path so that
 # we can include cmake files from this directory.
 list(INSERT CMAKE_MODULE_PATH 0
   "${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules"
   "${LLVM_COMMON_CMAKE_UTILS}/Modules"
   )
 
-if(LLVM_ENABLE_LIBXML2)
+# This allows disabling clang's XML dependency even if LLVM finds libxml2.
+# By default, clang depends on libxml2 if LLVM does.
+option(CLANG_ENABLE_LIBXML2 "Whether libclang may depend on libxml2"
+       ${LLVM_ENABLE_LIBXML2})
+
+if(CLANG_ENABLE_LIBXML2)
   # Don't look for libxml if we're using MSan, since uninstrumented third party
   # code may call MSan interceptors like strlen, leading to false positives.
   if(NOT LLVM_USE_SANITIZER MATCHES "Memory.*")
     set (LIBXML2_FOUND 0)
     find_package(LibXml2 2.5.3 QUIET)
     if (LIBXML2_FOUND)
       set(CLANG_HAVE_LIBXML 1)
     endif()