Extend debug info for function parameters in SDAG.

Extend debug info for function parameters in SDAG.

SDAG currently can emit debug location for function parameters when
an llvm.dbg.declare points to either a function argument SSA temp,
or to an AllocaInst. This change extends this logic by adding a
fallback case when neither of the above is true.

This is required for SafeStack, which may copy the contents of a
byval function argument into something that is not an alloca, and
then describe the target as the new location of the said argument.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@254352 91177308-0d34-0410-b5e6-96231b3b80d8