G++ wasn't emitting sufficient debug info for the debugger to be able to 
map capture names to the fields in the closure while inside the lambda 
body.  This patch changes the lambda implementation to generate proxy 
VAR_DECLs with DECL_VALUE_EXPR set so that they expand to the 
appropriate COMPONENT_REF for the capture field.  This way the debugger 
doesn't need to know about any special lookup rules, it just works.

Tested x86_64-pc-linux-gnu, applying to trunk.