Introduce serialization and deserialization of diagnostic information

Introduce serialization and deserialization of diagnostic information
so that CIndex can report diagnostics through the normal mechanisms
even when executing Clang in a separate process. This applies both
when performing code completion and when using ASTs as an intermediary
for clang_createTranslationUnitFromSourceFile().

The serialized format is not perfect at the moment, because it does
not encapsulate macro-instantiation information. Instead, it maps all
source locations back to the instantiation location. However, it does
maintain source-range and fix-it information. To get perfect fidelity
from the serialized format would require serializing a large chunk of
the source manager; at present, it isn't clear if this code will live
long enough for that to matter.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@94740 91177308-0d34-0410-b5e6-96231b3b80d8