SymbolVendorELF: Perform build-id lookup even without a debug link

Summary:
The debug link and build-id lookups are two independent ways one can
search for a separate symbol file. However, our implementation in
SymbolVendorELF was tying the two together and refusing to look up the
symbol file based on a build id if the file did not contain a debug
link.

This patch makes it possible to search for the symbol file with
just one of the two methods available. To demonstrate, I split the
build-id-case test into two, so that we test the search using both
methods.

Reviewers: jankratochvil, mgorny, clayborg, espindola, alexshap

Subscribers: emaste, arichardson, MaskRay, lldb-commits

Differential Revision: https://reviews.llvm.org/D65561

llvm-svn: 367994