You can use following commands to do the required task from the root directory of llvm source tree:
touch tags.lst
find | grep "\.c$" >> tags.lst
find | grep "\.cpp$" >> tags.lst
find | grep "\.h$" >> tags.lst
cscope -i tags.lst
It would create cscope.out file which is used with cscope to browse the code. Hope it helps!