Simplify codegen and debug info generation for block context parameters.

Simplify codegen and debug info generation for block context parameters.

The exisiting code goes out of its way to put block parameters into an
alloca only at -O0, and then describes the funciton argument with a
dbg.declare, which is undocumented in the LLVM-CFE contract and does
not actually behave as intended after LLVM r642022.

This patch just generates the alloca unconditionally, the mem2reg pass
will eliminate it at -O1 and up anyway and points the dbg.declare to
the alloca as intended (which mem2reg will then correctly rewrite into
a dbg.value).

rdar://problem/35043980

Differential Revision: https://reviews.llvm.org/D39305

llvm-svn: 316684