Tweak MemoryBuffer to allocate the class itself, the name and possibly the

buffer in the same chunk of memory.

2 less mallocs for every uninitialized MemoryBuffer and 1 less malloc for every
MemoryBuffer pointing to a memory range translate into 20% less mallocs on
clang -cc1 -Eonly Cocoa_h.m.

llvm-svn: 106839