[NativePDB] Add support for parsing typedef records.

Typedefs are represented as S_UDT records in the globals stream. This
creates a strange situation where "types" are actually represented as
"symbols", so they need special handling.

In order to test this, we don't just use lldb and print out some
variables causing the AST to get created, because variables whose type
is a typedef will have debug info referencing the original type, not the
typedef. So we use lldb-test instead which will parse all debug info in
the entire file. This exposed some problems with lldb-test and the
native reader, mainly that certain types of obscure symbols which we can
find when iterating every single record would trigger crashes. These
have been fixed as well so that lldb-test can be used to test this
functionality.

Differential Revision: https://reviews.llvm.org/D56461

llvm-svn: 350888