[CMake] Use find_dependency in LLVMConfig.cmake

[phosek]

Summary

This prefered over find_package as find_dependency forwards the correct
parameters for QUIET and REQUIRED to find_package.

[smeenai]

LGTM

llvm/cmake/modules/LLVMConfig.cmake.in

 set(LLVM_ENABLE_RTTI @LLVM_ENABLE_RTTI@)
 
 set(LLVM_ENABLE_TERMINFO @LLVM_ENABLE_TERMINFO@)
 
 set(LLVM_ENABLE_THREADS @LLVM_ENABLE_THREADS@)
 
 set(LLVM_ENABLE_UNWIND_TABLES @LLVM_ENABLE_UNWIND_TABLES@)
 
+include(CMakeFindDependencyMacro)
+
 set(LLVM_ENABLE_ZLIB @LLVM_ENABLE_ZLIB@)
 if(LLVM_ENABLE_ZLIB)
-  find_package(ZLIB)
+  find_dependency(ZLIB)
 endif()
 
 set(LLVM_ENABLE_LIBXML2 @LLVM_ENABLE_LIBXML2@)
 if(LLVM_ENABLE_LIBXML2)
-  find_package(LibXml2)
+  find_dependency(LibXml2)
 endif()
 
 set(LLVM_WITH_Z3 @LLVM_WITH_Z3@)
 
 set(LLVM_ENABLE_DIA_SDK @LLVM_ENABLE_DIA_SDK@)
 
 set(LLVM_NATIVE_ARCH @LLVM_NATIVE_ARCH@)