Patch 1/4 adding bitcode support.

Store whether or not a function is using Key Instructions in its DISubprogram so
that we don't need to rely on the -mllvm flag -dwarf-use-key-instructions to
determine whether or not to interpret Key Instructions metadata to decide
is_stmt placement at DWARF emission time. This makes bitcode support simple and
enables well defined mixing of non-key-instructions and key-instructions
functions in an LTO context.

This patch adds the bit (using DISubprogram::SubclassData1).

PR 144104 and 144103 use it during DWARF emission.
PR 44102 adds bitcode
support.

See pull request for overview of alternative attempts.

Seeing how we can't generate any debug intrinsics any more: delete a
variety of codepaths where they're handled. For the most part these are
plain deletions, in others I've tweaked comments to remain coherent, or
added a type to (what was) type-generic-lambdas.

This isn't all the DbgInfoIntrinsic call sites but it's most of the
simple scenarios.

Co-authored-by: Nikita Popov <github@npopov.com>

Change in #107278 modified the CMake CACHE variable with values
that are not supported for it as documented. This patch renames the
derived vars so that they do not conflict with the CACHE variable.

This patch transforms:

  X && *X == Y

to:

  X == Y

where X is of std::optional<T>, and Y is of T or similar.

RFC:
https://discourse.llvm.org/t/rfc-improving-is-stmt-placement-for-better-interactive-debugging/82668

Reapplied after fixing the config issue that was causing issues following
the previous merge.

This reverts commit fdbf073a86573c9ac4d595fac8e06d252ce1469f.

This reverts commit a9d93ecf1f8d2cfe3f77851e0df179b386cff353.

Reverted due to the commit including a config in LLVM headers that is not
available outside of the llvm source tree.

This is part of a series of patches that tries to improve DILocation bug
detection in Debugify; see the review for more details. This is the patch
that adds the main feature, adding a set of `DebugLoc::get<Kind>`
functions that can be used for instructions with intentionally empty
DebugLocs to prevent Debugify from treating them as bugs, removing the
currently-pervasive false positives and allowing us to use Debugify (in
its original DI preservation mode) to reliably detect existing bugs and
regressions. This patch does not add uses of these functions, except for
once in Clang before optimizations, and in
`Instruction::dropLocation()`, since that is an obvious case that
immediately removes a set of false positives.

The applyDebugify helpers were providing default arguments despite all callers providing them, so strip them for clarity.

The Function variant was asserting that DebugInfoBeforePass was non-null before dereferencing so I've added an equivalent assert to the Method variant as well.

Fixes #97626

After #124287 updated several functions to return iterators rather than
Instruction *, it was no longer straightforward to pass their result to
DIBuilder. This commit updates DIBuilder methods to accept an
InsertPosition instead, so that they can be called with an iterator
(preferred), or with a deprecation warning an Instruction *, or a
BasicBlock *. This commit also updates the existing calls to the
DIBuilder methods to pass in iterators.

As a special exception, DIBuilder::insertDeclare() keeps a separate
overload accepting a BasicBlock *InsertAtEnd. This is because despite
the name, this method does not insert at the end of the block, therefore
this cannot be handled implicitly by using InsertPosition.