and adds a new file ReaderWriterYAML.cpp that uses YAML I/O.

Lots of tweaks to test suite for slightly different YAML encoding.

llvm-svn: 171592

shared library) and AbsoluteAtoms (proxy atoms for absolute address (e.g. ROM)).
Redesign weak importing as can-be-null-at-runtime and can-be-null-at-build-time.
Add lots of test cases for all the above.

llvm-svn: 151204