PR14296: function parameter name collisions in function try/catch

C++11 3.3.3/2 "A parameter name shall not be redeclared in the outermost block
of the function definition nor in the outermost block of any handler associated
with a function-try-block."

It's not totally clear to me whether the "FIXME" case is covered by this, but
Richard Smith thinks it probably should be. It's just a bit more involved to
fix that case.

llvm-svn: 167650