While I am at it, this patch switches to the constructor that takes
a container instead of a pair of begin/end.

Identified with readability-const-return-type.

Skip creating a module if no function is going to be imported.
Also includes a change so that if the first partition is empty (which
can happen),
we import global with non-local linkage into the first non-empty
partition, instead
of P0 all the time.

I thought we'd need to change users of the SplitModule callback so they
can deal with less modules
than the number requested, but no. We already return only 1 module in
some cases and
it seems to be handled just fine.

Fixes SWDEV-523146

Fix a reference to getValue() being optional in InlineSizeEstimatorAnalysis, a
file that is not included in the default build. A "warning: enumerated and
non-enumerated type in conditional expression" warning is fixed in AMDGPU too.

InstructionCost is already an optional value, containing an Invalid
state that can be checked with isValid(). There is little point in
returning another optional from getValue(). Most uses do not make use of
it being a std::optional, dereferencing the value directly (either
isValid has been checked previously or the Cost is assumed to be valid).
The one case that does in AMDGPU used value_or which has been replaced
by a isValid() check.

Introduce members() iterator-helper to shorten the members_{begin,end}
idiom. A previous attempt of this patch was #130319, which had to be
reverted due to unit-test failures when attempting to call members() on
the end iterator. In this patch, members() accepts either an ECValue or
an ElemTy, which is more intuitive and doesn't suffer from the same
issue.

Currently iterators over EquivalenceClasses will iterate over std::set,
which guarantees the order specified by the comperator. Unfortunately in
many cases, EquivalenceClasses are used with pointers, so iterating over
std::set of pointers will not be deterministic across runs.

There are multiple places that explicitly try to sort the equivalence
classes before using them to try to get a deterministic order
(LowerTypeTests, SplitModule), but there are others that do not at the
moment and this can result at least in non-determinstic value naming in
Float2Int.

This patch updates EquivalenceClasses to keep track of all members via a
extra SmallVector and removes code from LowerTypeTests and SplitModule
to sort the classes before processing.

Overall it looks like compile-time slightly decreases in most cases, but
close to noise:

https://llvm-compile-time-tracker.com/compare.php?from=7d441d9892295a6eb8aaf481e1715f039f6f224f&to=b0c2ac67a88d3ef86987e2f82115ea0170675a17&stat=instructions

PR: https://github.com/llvm/llvm-project/pull/134075

Remove a level of indirection and update code to use range-based for
loops.

DenseSet, SmallPtrSet, SmallSet, SetVector, and StringSet recently
gained C++23-style insert_range.  This patch replaces:

  Dest.insert(Src.begin(), Src.end());

with:

  Dest.insert_range(Src);

This patch does not touch custom begin like succ_begin for now.

This PR abstracts the `LockFileManager` API into new `AdvisoryLock`
interface. This is so that we can create an alternative implementation
for Clang implicitly-built modules that is optimized for single-process
environment.

Clients of `LockFileManager` call `unsafeRemoveLockFile()` whenever
`tryLock()` fails. However looking at the code, there are no scenarios
where this actually does something useful. This PR removes such calls.