[PDB] Write public symbol records and the publics hash table

[PDB] Write public symbol records and the publics hash table

Summary:
MSVC link.exe records all external symbol names in the publics stream.
It provides similar functionality to an ELF .symtab.

Reviewers: zturner, ruiu

Subscribers: hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D35871

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@309303 91177308-0d34-0410-b5e6-96231b3b80d8