Teach the default symbol vendor to respect module.GetSymbolFileFileSpec()

Summary:
Adding a breakpad symbol file to an existing MachO module with "target symbols
add" currently works only if one's host platform is a mac. This is
because SymbolVendorMacOSX (which is the one responsible for loading
symbols for MachO files) is conditionally compiled for the mac platform.

While we will sooner or later have a special symbol vendor for breakpad
files (to enable more advanced searching), and so this flow could be
made to work through that, it's not clear to me whether this should be a
requirement for the "target symbols add" flow to work. After all, since
the user has explicitly specified the symbol file to use, the symbol
vendor plugin's job is pretty much done.

This patch teaches the default symbol vendor to respect module's symbol
file spec, and load the symbol from that file if it is specified (and no
plugin requests any special handling).

Reviewers: clayborg, zturner, lemo

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D56589

llvm-svn: 351330