Simplified Serialization code for SourceLocation and SourceRange, and

Simplified Serialization code for SourceLocation and SourceRange, and
updated it to the recently updated Serialization API.

Changed clients of SourceLocation serialization to call the
appropriate new methods.

Updated Decl serialization code to put new skeleton serialization code
in place that is much better than the older trait-specialization
approach.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@43625 91177308-0d34-0410-b5e6-96231b3b80d8