[analyzer] operator new: Fix callback order for CXXNewExpr.

PreStmt<CXXNewExpr> was never called.

Additionally, under c++-allocator-inlining=true, PostStmt<CXXNewExpr> was
called twice when the allocator was inlined: once after evaluating the
new-expression itself, once after evaluating the allocator call which, for the
lack of better options, uses the new-expression as the call site.

This patch fixes both problems.

Differential Revision: https://reviews.llvm.org/D41934
rdar://problem/12180598

llvm-svn: 322797