[lldb-test] Modify lldb-test to print out ASTs from symbol file

Summary:
Currently when invoking lldb-test symbols -dump-ast it parses all the debug symbols and calls print(...) on the TranslationUnitDecl.
While useful the TranslationUnitDecl::print(...) method gives us a higher level view then the dump from ASTDumper which is what we get when we invoke dump() on a specific AST node.
The main motivation for this change is allow us to verify that the AST nodes we create when we parse DWARF. For example in order to verify we are correctly using DIFlagExportSymbols added by D66667

Differential Revision: https://reviews.llvm.org/D67994

llvm-svn: 374570