[CMake] Fix find_package for libXml2 on case sensitive filesystems.

[rjmansfield]

This fixes the find_package to work on a case sensitive file system when LLVM_ENABLE_LIBXML2 is true. The cmake finder is FindLibXml2.cmake. Otherwise, if set, the build will fail with:

CMake Error at CMakeLists.txt:929 (find_package):
By not providing "FindLibxml2.cmake" in CMAKE_MODULE_PATH this project has
asked CMake to find a package configuration file provided by "Libxml2", but
CMake did not find one.

Could not find a package configuration file provided by "Libxml2" with any
of the following names:

Libxml2Config.cmake
libxml2-config.cmake

@swift-ci Please smoke test

[CodaFi]

Aren’t strings as a universal data type just the greatest.

Thanks for the patch!

@swift-ci please smoke test