change the StringLiteral AST node to track all of the SourceLocations of

change the StringLiteral AST node to track all of the SourceLocations of
the various PPTokens that are pasted together to make it.  In the course
of working on this, I discovered ParseObjCStringLiteral which needs some
work.  I'll tackle it next.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@64892 91177308-0d34-0410-b5e6-96231b3b80d8