[LTO][Legacy] Add new C inferface to query libcall functions

[LTO][Legacy] Add new C inferface to query libcall functions

Summary:
This is needed to implemented the same approach as lld (implemented in r338434)
for how to handling symbols that can be generated by LTO code generator
but not present in the symbol table for linker that uses legacy C APIs.

libLTO is in charge of providing the list of symbols. Linker is in
charge of implementing the eager loading from static libraries using
the list of symbols.

rdar://problem/52853974

Reviewers: tejohnson, bd1976llvm, deadalnix, espindola

Reviewed By: tejohnson

Subscribers: emaste, arichardson, hiraditya, MaskRay, dang, kledzik, mehdi_amini, inglorion, jkorous, dexonsmith, ributzka, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67568

llvm-svn: 372021