Fix regex & startsWith name lookup in SBTarget::FindGlobalVariables

There were two bugs here.

eMatchTypeStartsWith searched for "symbol_name" by adding ".*" to the
end of the symbol name and treating that as a regex, which isn't
actually a regex for "starts with". The ".*" is in fact a no-op.  When
we finally get to comparing the name, we compare against whatever form
of the name was in the accelerator table. But for C++ that might be
the mangled name. We should also try demangled names here, since most
users are going the see demangled not mangled names.  I fixed these
two bugs and added a bunch of tests for FindGlobalVariables.

This change is in the DWARF parser code, so there may be a similar bug
in PDB, but the test for this was already skipped for Windows, so I
don't know about this.

You might theoretically need to do this Mangled comparison in

DWARFMappedHash::MemoryTable::FindByName

except when we have names we always chop them before looking them up
so I couldn't see any code paths that fail without that change. So I
didn't add that to this patch.

Differential Revision: https://reviews.llvm.org/D151940