Revise debug info machinery to digest nested functions and classes.

A certain GDB testsuite case (local.cc) has a function nested inside a
class nested inside another function.  GCC presents the innermost
function to llvm-convert first.  Heretofore, the debug info mistakenly
placed the inner function at module scope.  This patch walks the GCC
context links and instantiates the outer class and function so the
debug info is properly nested.  Radar 7426545.

llvm-svn: 100530