Fix capitalization of LibXml2 for CMake on case-sensitive file systems

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@161445 91177308-0d34-0410-b5e6-96231b3b80d8

CMakeLists.txt

@@ -141,7 +141,7 @@ endif ()
 
 # libxml2 is an optional dependency, required only to run validation
 # tests on XML output.
-find_package(libxml2)
+find_package(LibXml2)
 
 configure_file(
   ${CLANG_SOURCE_DIR}/include/clang/Config/config.h.cmake