Fix a problem with objc foreach loop. It turns out that objc mode changes

for scoping to match C99 even when in C89 mode.  This patch fixes this 
(eliminating a "redefinition of thisKey" error), and also prevents non-sensical
diagnostics in -pedantic mode like this:

t.m:7:8: warning: variable declaration in for loop is a C99-specific feature
  for (id thisKey in keys) ;
       ^

llvm-svn: 69760