[DebugInfo] Keep parameter DBG_VALUEs before prologue code

Summary:
This is a preparatory change for removing the code from
DebugHandlerBase::beginFunction() which changes the starting label for
the first non-overlapping DBG_VALUEs of parameters to the beginning of
the function. It does that to be able to show parameters when entering a
function. However, that code does not consider what defines the values,
which can result in the ranges for the debug values starting before
their defining instructions. That code is removed in a follow-up patch.

When prologue code is inserted, it leads to DBG_VALUEs that start
directly in the entry block being moved down after the prologue
instructions. This patch fixes that by stashing away DBG_VALUEs for
parameters before emitting the prologue, and then reinserts them at the
start of the block. This assumes that there is no target that somehow
clobbers parameter registers in the frame setup; there is no such case
in the lit tests at least.

See PR40188 for more information.

Reviewers: aprantl, dblaikie, rnk, jmorse

Reviewed By: aprantl

Subscribers: bjope, llvm-commits

Tags: #debug-info

Differential Revision: https://reviews.llvm.org/D57510

llvm-svn: 353823