A cast from DIExpression->DIExpression is not needed.

Differential Revision: https://reviews.llvm.org/D149178

Prior to this patch, variadic DIExpressions (i.e. ones that contain
DW_OP_LLVM_arg) could only be created by salvaging debug values to create
stack value expressions, resulting in a DBG_VALUE_LIST being created. As of
the previous patch in this patch stack, DBG_INSTR_REF's syntax has been
changed to match DBG_VALUE_LIST in preparation for supporting variadic
expressions. This patch adds some minor changes needed to allow variadic
expressions that aren't stack values to exist, and allows variadic expressions
that are trivially reduceable to non-variadic expressions to be handled
similarly to non-variadic expressions.

Reviewed by: jmorse

Differential Revision: https://reviews.llvm.org/D133926

This patch adds a new function that can be used to check all the
properties, other than the machine values, of a pair of debug values for
equivalence. This is done by folding the "directness" into the
expression, converting the expression to variadic form if it is not
already in that form, and then comparing directly. In a few places which
check whether two debug values are identical to see if their ranges can
be merged, this function will correctly identify cases where two debug
values are expressed differently but have the same meaning, allowing
those ranges to be correctly merged.

Differential Revision: https://reviews.llvm.org/D136173

Identified with readability-const-return-type.

DebugLocEntry assumes that it either contains 1 item that has no fragment
or many items that all have fragments (see the assert in addValues).

When EXPENSIVE_CHECKS is enabled, _GLIBCXX_DEBUG is defined. On a few machines
I've checked, this causes std::sort to call the comparator even
if there is only 1 item to sort. Perhaps to check that it is implemented
properly ordering wise, I didn't find out exactly why.

operator< for a DbgValueLoc will crash if this happens because the
optional Fragment is empty.

Compiler/linker/optimisation level seems to make this happen
or not. So I've seen this happen on x86 Ubuntu but the buildbot
for release EXPENSIVE_CHECKS did not have this issue.

Add an explicit check whether we have 1 item.

Reviewed By: aprantl

Differential Revision: https://reviews.llvm.org/D130156

Avoid unnecessary copies, reported by MSVC static analyzer.

This is a partial reapply of the original commit and the followup commit
that were previously reverted; this reapply also includes a small fix
for a potential source of non-determinism, but also has a small change
to turn off variadic debug value salvaging, to ensure that any future
revert/reapply steps to disable and renable this feature do not risk
causing conflicts.

Differential Revision: https://reviews.llvm.org/D91722

This reverts commit 386b66b2fc297cda121a3cc8a36887a6ecbcfc68.

> This reapplies c0f3dfb9, which was reverted following the discovery of
> crashes on linux kernel and chromium builds - these issues have since
> been fixed, allowing this patch to re-land.

This reverts commit 36ec97f76ac0d8be76fb16ac521f55126766267d.

The change caused non-determinism in the compiler, see comments on the code
review at https://reviews.llvm.org/D91722.

Reverting to unbreak people's builds until that can be addressed.

This also reverts the follow-up "[DebugInfo] Limit the number of values
that may be referenced by a dbg.value" in
a0bd6105d80698c53ceaa64bbe6e3b7e7bbf99ee.