[analyzer] Create a temp region when a method is called on a struct rvalue.

An rvalue has no address, but calling a C++ member function requires a
'this' pointer. This commit makes the analyzer create a temporary region
in which to store the struct rvalue and use as a 'this' pointer whenever
a member function is called on an rvalue, which is essentially what
CodeGen does.

More of <rdar://problem/12137950>. The last part is tracking down the
C++ FIXME in array-struct-region.cpp.

llvm-svn: 164829