[ADT] Allow K to be incomplete during DenseMap<K*, V> instantiation

DenseMap requires two sentinel values for keys: empty and tombstone
values. To avoid undefined behavior, LLVM aligns the two sentinel
pointers to alignof(T). This requires T to be complete, which is
needlessly restrictive.

Instead, assume that DenseMap pointer keys have a maximum alignment of
4096, and use the same sentinel values for all pointer keys. The new
sentinels are:
  empty:     static_cast<uintptr_t>(-1) << 12
  tombstone: static_cast<uintptr_t>(-2) << 12

These correspond to the addresses of -4096 and -8192. Hopefully, such a
key is never inserted into a DenseMap.

I encountered this while looking at making clang's SourceManager not
require FileManager.h, but it has several maps keyed on classes defined
in FileManager.h. FileManager depends on various LLVM FS headers, which
cumulatively take ~200ms to parse, and are generally not needed.

Reviewed By: hans

Differential Revision: https://reviews.llvm.org/D75301