Fix LLVMConfig.cmake being generated to use correct LLVM_INSTALL_PREFIX
(e.g. /usr/lib/llvm5).

This is needed e.g. for building lldb.

--- a/cmake/modules/CMakeLists.txt
+++ b/cmake/modules/CMakeLists.txt
@@ -75,15 +75,7 @@
 
 # Generate LLVMConfig.cmake for the install tree.
 set(LLVM_CONFIG_CODE "
-# Compute the installation prefix from this LLVMConfig.cmake file location.
-get_filename_component(LLVM_INSTALL_PREFIX \"\${CMAKE_CURRENT_LIST_FILE}\" PATH)")
-# Construct the proper number of get_filename_component(... PATH)
-# calls to compute the installation prefix.
-string(REGEX REPLACE "/" ";" _count "${LLVM_INSTALL_PACKAGE_DIR}")
-foreach(p ${_count})
-  set(LLVM_CONFIG_CODE "${LLVM_CONFIG_CODE}
-get_filename_component(LLVM_INSTALL_PREFIX \"\${LLVM_INSTALL_PREFIX}\" PATH)")
-endforeach(p)
+set(LLVM_INSTALL_PREFIX \"${CMAKE_INSTALL_PREFIX}\")")
 set(LLVM_CONFIG_INCLUDE_DIRS "\${LLVM_INSTALL_PREFIX}/include")
 set(LLVM_CONFIG_LIBRARY_DIRS "\${LLVM_INSTALL_PREFIX}/lib\${LLVM_LIBDIR_SUFFIX}")
 set(LLVM_CONFIG_CMAKE_DIR "\${LLVM_INSTALL_PREFIX}/${LLVM_INSTALL_PACKAGE_DIR}")