it returns a list of functions as a SymbolContextList.

Rewrote the clients of SymbolContext to use this
SymbolContextList.

Rewrote some of the providers of the data to SymbolContext
to make them respect preferences as to whether the list
should be cleared first; propagated that change out.

ClangExpressionDeclMap and ClangASTSource use this new
function list to properly generate function definitions -
even for functions that don't have a prototype in the
debug information.

llvm-svn: 109476