Grow BumpPtrAllocator's slab size dynamically if we allocated many slabs. This

Grow BumpPtrAllocator's slab size dynamically if we allocated many slabs. This
reduces the amount of malloc calls and may reduce memory overhead.

Some numbers:
ASTContext stats, clang -cc1 -disable-free -fsyntax-only Cocoa_h.m
without dynamic growth                          |  with dynamic growth
Number of memory regions: 3158                  |  Number of memory regions: 432
Bytes used: 12333185                            |  Bytes used: 12333185
Bytes allocated: 12935168                       |  Bytes allocated: 12800000
Bytes wasted: 601983 (includes alignment, etc)  |  Bytes wasted: 466815 (includes alignment, etc)

ASTContext stats, clang -cc1 -disable-free -fsyntax-only on clang's ASTReader.cpp
without dynamic growth                          |  with dynamic growth
Number of memory regions: 10987                 |  Number of memory regions: 551
Bytes used: 42910356                            |  Bytes used: 42910356
Bytes allocated: 45002752                       |  Bytes allocated: 44711936
Bytes wasted: 2092396 (includes alignment, etc) |  Bytes wasted: 1801580 (includes alignment, etc)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@115151 91177308-0d34-0410-b5e6-96231b3b80d8