Cleanup to simplify the formatter for std::map of libstdc++.

Summary:
GCC does not emit some DWARF required for the simplified formatter
to work. A workaround for it has been incorporated in the formatter.
The corresponding test TestDataFormatterStdMap has also been enabled
for GCC.

Test Plan: dotest.py -C <clang|gcc> -p TestDataFormatterStdMap

Reviewers: clayborg, vharron, granata.enrico

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D8424

llvm-svn: 232678