Miscellaneous fixes for big-endian systems

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.

Differential Revision: http://reviews.llvm.org/D18983

llvm-svn: 266313