This patch fixes a bunch of issues that show up on big-endian systems:
- The gnu_libstdcpp.py script doesn't follow the way libstdc++ encodes bit vectors: it should identify the enclosing *word* and then access the appropriate bit within that word. Instead, the script simply operates on bytes. This gives the same result on little-endian systems, but not on big-endian.
- lldb_private::formatters::WCharSummaryProvider always assumes wchar_t is UTF16, even though it could also be UTF8 or UTF32. This is mostly not an issue on little-endian systems, but immediately fails on BE. Fixed by checking the size of wchar_t like WCharStringSummaryProvider already does.
- ClangASTContext::GetChildCompilerTypeAtIndex uses uint32_t to access the virtual base offset stored in the vtable, even though the size of this field matches the target pointer size according to the C++ ABI. Again, this is mostly not visible on LE, but fails on BE.
- Process::ReadStringFromMemory uses strncmp to search for a terminator consisting of multiple zero bytes. This doesn't work since strncmp will stop already at the first zero byte. Use memcmp instead.