Remove SymbolVendor::GetSymtab

Summary:
This patch removes the GetSymtab method from the SymbolVendor, which is
a no-op as it's implementation just forwards to the relevant SymbolFile.
Instead it creates a Module::GetSymtab, which calls the SymbolFile
method directly.

All callers have been updated to use the Module method directly instead
of a two phase GetSymbolVendor->GetSymtab search, which leads to reduced
intentation in a lot of deeply nested code.

Reviewers: clayborg, JDevlieghere, jingham

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D65569

llvm-svn: 367820