Determine endianness at a time when it doesn't inadvertantly clear gdb's...

Determine endianness at a time when it doesn't inadvertantly clear gdb's wrap_buffer via gdb.execute.

Summary:
I haven't managed a small reproduction for this bug, it involves
complicated and deeply nested data structures with a wide variety
of pretty printers. But in general, we shouldn't be combining
gdb's command line interface (via gdb.execute) with pretty-printers.

Subscribers: christof, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68306

llvm-svn: 373402