[dsymutil] Accept a YAML debug map as input instead of a binary.

To do this, the user needs to pass the new -y flag.
As it wasn't tested before, the debug map YAML deserialization was
completely buggy (mainly because the DebugMapObject has a dual
mapping that allows to search by name and by address, but only the
StringMap got populated). It's fixed and tested in this commit by
augmenting some test with a 2 stage dwarf link: a frist llvm-dsymutil
reads the debug map and pipes it in a second instance that does the
actual link without touching the initial binary.

llvm-svn: 238941