Sema: Ban forward references of captured values from 'defer' body
While 'defer' is implemented as a local function, it doesn't
behave as one. In particular, since SILGen runs it after
destroying all local bindings that appear after the 'defer'
definition, the body of a 'defer' cannot forward reference
captured bindings the way that local functions can.
Note that I had to remove a SILGen test case for an older,
related issue. The new diagnostic in Sema catches these cases
earlier.
Fixes rdar://problem/75088379.
