[Sanitizers] Increase allocated chunk limit for LargeMmapAllocator

Summary:
There are applications out there which allocate more than 1 << 18 large chunks
of memory (those handled by LargeMmapAllocator, aka secondary allocator).

For 64 bits, secondary allocator stores allocated chunks in a growing on
demand region of memory, growing in blocks of 128K, up to 1 << 20 chunks total.

Sanitizer internal allocator's secondary uses fixed size array storing up
to 1 << 15 chunks (down to 256K from 2Mb of memory used for that array).

Nothing is changed for 32 bits, chunks are still stored in the fixed size
array (up to 1 << 15 chunks).

Reviewers: eugenis

Subscribers: kubamracek, delcypher, #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D43693

llvm-svn: 326007