[llvm][dwwarf] Change CU/TU index to 64-bit

Changed contribution data structure to 64 bit. I added the 32bit and 64bit
accessors to make it explicit where we use 32bit and where we use 64bit. Also to
make sure sure we catch all the cases where this data structure is used.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D139379