Make llvm-rtdlyd -check preserve automatic address mappings made by RuntimeDyld.

Currently llvm-rtdyld in -check mode will map sections to back-to-back 4k
aligned slabs starting at 0x1000. Automatically remapping sections by default is
helpful because it quickly exposes relocation bugs due to use of local addresses
rather than load addresses (these would silently pass if the load address was
not remapped). These mappings can be explicitly overridden on a per-section
basis using llvm-rtdlyd's -map-section option. This patch extends this scheme to
also preserve any mappings made by RuntimeDyld itself. Preserving RuntimeDyld's
automatic mappings allows us to write test cases to verify that these automatic
mappings have been applied.

This will allow the fix in https://reviews.llvm.org/D32899 to be tested with
llvm-rtdyld -check.

llvm-svn: 302372