Add a -verify-debug-info option that invokes dwarfdump --verify as the last step after running dsymutil. dwarfdump is invoked with same options clang 802.0.35 uses to invoke it:
dwarfdump --verify --debug-info --eh-frame --quiet
A warning is produced if -verify-debug-info is set and no debug option is also set.
dwarfdump is failing to validate the debug info in the test verify-debug-info.swift. The failure is:
error: .debug_line[0x0000007d].row[0].file = 1 is not a valid index
https://bugs.swift.org/browse/SR-2396
