Assume that mergeable input sections are smaller than 4 GiB.

By assuming that mergeable input sections are smaller than 4 GiB,
lld's memory usage when linking clang with debug info drops from
2.788 GiB to 2.019 GiB (measured by valgrind, and that does not include
memory space for mmap'ed files). I think that's a reasonable assumption
given such a large RAM savings, so this patch.

According to valgrind, gold needs 3.54 GiB of RAM to do the same thing.

NB: This patch does not introduce a limitation on the size of
output sections. You can still create sections larger than 4 GiB.

llvm-svn: 316280