[dsymutil] Ignore absolute symbols in the debug map

Quoting from the comment added to the code:

    // Objective-C on i386 uses artificial absolute symbols to
    // perform some link time checks. Those symbols have a fixed 0
    // address that might conflict with real symbols in the object
    // file. As I cannot see a way for absolute symbols to find
    // their way into the debug information, let's just ignore those.

llvm-svn: 255350