[Support] Split MallocAllocator out of Allocator.h

StringMap.h is very popular (4K uses), and it doesn't need to see
BumpPtrAllocator, which is relatively expensive according to
ClangBuildAnalyzer. StringMap only needs MallocAllocator, so split that
into AllocatorBase.h and use it instead.

Here is the change in header uses:
$ diff -u thedeps-before.txt thedeps-after.txt | \
    grep '^[-+] ' |  sort | uniq -c | sort -nr
   3993 +    ../llvm/include/llvm/Support/AllocatorBase.h
    758 -    ../llvm/include/llvm/Support/Allocator.h
    270 -    ../llvm/include/llvm/Support/Alignment.h
     13 -    ../llvm/include/llvm/Support/Host.h
      6 -    ../llvm/include/llvm/ADT/StringMap.h
      4 -    ../llvm/include/llvm/Support/SwapByteOrder.h
      4 -    ../llvm/include/llvm/Support/MathExtras.h
      4 -    ../llvm/include/llvm/Support/AlignOf.h
      4 -    ../llvm/include/llvm/ADT/SmallVector.h
      1 -    ../llvm/include/llvm/Support/PointerLikeTypeTraits.h

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D73392