Optimize PartialDiagnostic's memory-allocation behavior by placing a

cache of PartialDiagnostic::Storage objects into an allocator within
the ASTContext. This eliminates a significant amount of malloc
traffic, for a 10% performance improvement in -fsyntax-only wall-clock
time with 403.gcc's combine.c.

Also, eliminate the RequireNonAbstractType hack I put in earlier,
which was but a symptom of this larger problem.

Fixes <rdar://problem/7806091>.

llvm-svn: 99849