Change Metadata Index emission in the bitcode to use 2x32 bits for the placeholder

The Bitstream reader and writer are limited to handle a "size_t" at
most, which means that we can't backpatch and read back a 64bits
value on 32 bits platform.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@290693 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed