The DWARF data for Swift code has a top-level DW_TAG_module DIE as the
child of the DW_TAG_compile_unit DIE and the parent of the
DW_TAG_subprogram DIEs that dump_syms uses to locate functions.
dump_syms needs to process DW_TAG_module DIEs as introducing nested
scopes to make it work with Swift.

This also reworks demangling to be language-specific, so that the C++
demangler isn't invoked when processing Swift code. The DWARF data for
Swift code presents its mangled names in the same form as used for C++
(DW_AT_MIPS_linkage_name or DW_AT_linkage_name) but the mangling is
Swift-specific (beginning with _T instead of _Z). There is no
programmatic interface to a Swift name demangler as an analogue to C++'s
__cxa_demangle(), so mangled Swift names are exposed as-is. Xcode's
"xcrun swift-demangle" can be used to post-process these mangled Swift
names on macOS.

Support for mangled names presented in a DW_AT_linkage_name attribute,
as used by DWARF 4...