Remove Module::GetSymbolVendor

Summary:
This patch removes the GetSymbolVendor function, and the various
mentions of the SymbolVendor in the Module class. The implementation of
GetSymbolVendor is "inlined" into the GetSymbolFile class which I
created earlier.

After this patch, the SymbolVendor class still exists inside the Module
object, but only as an implementation detail -- a fancy holder for the
SymbolFile. That will be removed in the next patch.

Reviewers: clayborg, JDevlieghere, jingham, jdoerfert

Subscribers: jfb, lldb-commits

Differential Revision: https://reviews.llvm.org/D65864

llvm-svn: 368263