[NativePDB] Add basic support for tag types to the native pdb plugin.

This adds support to LLDB for named types (class, struct, union, and
enum).  This is true cross platform support, and hits the PDB file
directly without a dependency on Windows.  Tests are added which
compile a program with certain interesting types and then use
load the target in LLDB and use "type lookup -- <TypeName>" to
dump the layout of the type in LLDB without a running process.

Currently only fields are parsed -- we do not parse methods.  Also
we don't deal with bitfields or virtual bases correctly.  Those
will make good followups.

Differential Revision: https://reviews.llvm.org/D53511

llvm-svn: 345047