This patch replaces LLVM_ATTRIBUTE_UNUSED with [[maybe_unused]],
introduced as part of C++17.

`replaceAllUsesWith` is not safe to use in a dialect conversion and will
be deactivated soon (#154112). Fix commit fixes some API violations.
Also some general improvements.

Now that #149310 has restricted lifetime intrinsics to only work on
allocas, we can also drop the explicit size argument. Instead, the size
is implied by the alloca.

This removes the ability to only mark a prefix of an alloca alive/dead.
We never used that capability, so we should remove the need to handle
that possibility everywhere (though many key places, including stack
coloring, did not actually respect this).

See https://github.com/llvm/llvm-project/pull/147168 for more info.

Flang at Ofast usually produces executables that consume more stack that
other Fortran compilers.
This is in part because the alloca created from temporary heap
allocation by the StackArray pass are created at the function scope
level without lifetimes, and LLVM does not/is not able to merge alloca
that do not have overlapping lifetimes.

This patch adds an option to generate LLVM lifetime in the StackArray
pass at the previous heap allocation/free using the LLVM dialect
operation for it.

This is similar to other configuration objects used across MLIR.

Rename some fields to better reflect that they are no longer booleans.

Reland 04d261101b4f229189463136a794e3e362a793af / #132253.

This reverts commit 63b8f1c9482ed0a964980df4aed89bef922b8078.

Buildbot failure:
https://lab.llvm.org/buildbot/#/builders/172/builds/12083/steps/5/logs/stdio

I've reproduced the error with a release build (-DCMAKE_BUILD_TYPE=Release).

This is similar to other configuration objects used across MLIR.

`findAllocaLoopInsertionPoint()` hit assertion not being able
to find the `fir.freemem` because of the `fir.convert`.
I think it is better to look for `fir.freemem` same way
with the look-through walk.

This does add a little computational complexity because now every
freemem operation has to be tested for every allocation. This could be
improved with some more memoisation but I think it is easier to read
this way. Let me know if you would prefer me to change this to
pre-compute the normalised addresses each freemem operation is using.

Weirdly, this change resulted in a verifier failure for the fir.declare
in the previous test case. Maybe it was previously removed as dead code
and now it isn't. Anyway I fixed that too.