MetadataReader: Add an API for reading absolute pointers.
Pointer data in some remote reflection targets may required relocation, or may not be
fully resolvable, such as when we're dumping info from a single image on disk that
references other dynamic libraries. Add a `RemoteAbsolutePointer` type that can hold a
symbol, offset, or combination of both, and add APIs to `MemoryReader` and `MetadataReader`
for reading pointers that can get unresolved relocation info from an image, or apply
relocations to pointer information. MetadataReader can use the symbol name information to
fill in demanglings of symbolic-reference-bearing mangled names by using the information
from the symbol name to fill in the name even though the context descriptors are not
available.
For now, this is NFC (MemoryReader::resolvePointer just forwards the pointer data), but
lays the groundwork for implementation of relocation in ObjectMemoryReader.
