MC, Object: Reserve a section type, SHT_LLVM_ODRTAB, for the ODR table.

This is part of the ODR checker proposal:
http://lists.llvm.org/pipermail/llvm-dev/2017-June/113820.html

Per discussion on the gnu-gabi mailing list [1] the section type range
0x6fff4c00..0x6fff4cff is reserved for LLVM.

[1] https://sourceware.org/ml/gnu-gabi/2017-q2/msg00030.html

Differential Revision: https://reviews.llvm.org/D33978

llvm-svn: 305407