Fix PDB tests after r367820

The commit changed Module dumping code to call SymbolFile::Dump
directly, which meant that we were no longer showing the plugin name in
the output (as that was done in the SymbolVendor).

This adds the plugin name printing code to the SymbolFile dump method,
and tweak the assertions in the PDB tests to match it correctly.

llvm-svn: 367835