[ELF] Read the call graph profile from object files.

This uses the call graph profile embedded in the object files to construct the call graph.

This is read from a SHT_LLVM_CALL_GRAPH_PROFILE (0x6fff4c02) section as (uint32_t, uint32_t, uint64_t) tuples as (from symbol index, to symbol index, weight).

Differential Revision: https://reviews.llvm.org/D45850

llvm-svn: 343552