Extend RTDyld API to enable optionally precomputing the total amount of memory

Extend RTDyld API to enable optionally precomputing the total amount of memory
required for all sections in a module. This can be useful when targets or
code-models place strict requirements on how sections must be laid out
in memory.

If RTDyldMemoryManger::needsToReserveAllocationSpace() is overridden to return
true then the JIT will call the following method on the memory manager, which
can be used to preallocate the necessary memory.

void RTDyldMemoryManager::reserveAllocationSpace(uintptr_t CodeSize,
                                                 uintptr_t DataSizeRO,
                                                 uintptr_t DataSizeRW)

Patch by Vaidas Gasiunas. Thanks very much Viadas!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201259 91177308-0d34-0410-b5e6-96231b3b80d8