In functions that have been seriously deformed during optimisation,
there can be call instructions with line-zero immediately after frame
setup (see C reproducer in the test added). Our previous algorithms for
prologue_end ignored these, meaning someone entering a function at
prologue_end would break-in after a function call had completed. Prefer
instead to place prologue_end and the function scope-line on the line
zero call: this isn't false (it's the first meaningful instruction of the
function) and is approximately true. Given a less than ideal function,
this is an OK solution.

This patch is just a small cleanup that unifies the various spots that
add a DWARF expression to the output.

Per https://llvm.org/docs/CodingStandards.html#include-as-little-as-possible
this improves compilation time, while not being too intrusive on the
codebase.

Per https://llvm.org/docs/CodingStandards.html#include-as-little-as-possible this improves compilation time, while not being too intrusive on the codebase.

These checks ensure that retained nodes of a DISubprogram belong to the
subprogram.

Tests with incorrect IR are fixed. We should not have variables of one subprogram present in retained nodes of other subprograms.

Also, interface for accessing DISubprogram's retained nodes is slightly
refactored. `DISubprogram::visitRetainedNodes` and
`DISubprogram::forEachRetainedNode` are added to avoid repeating checks
like
```
if (const auto *LV = dyn_cast<DILocalVariable>(N))
  ...
else if (const auto *L = dyn_cast<DILabel>(N))
  ...
else if (const auto *IE = dyn_cast<DIImportedEntity>(N))
  ...
```

This intrinsic emits a BFD_RELOC_NONE relocation at the point of call,
which allows optimizations and languages to explicitly pull in symbols
from static libraries without there being any code or data that has an
effectual relocation against such a symbol.

See issue #146159 for context.

In the call graph section, we were emitting the temporary label
pointing to the start of the function instead of the canonical linkage
correct function symbol. This patch fixes it and updates the
corresponding tests.

In C++17, static constexpr members are implicitly inline, so they no
longer require an out-of-line definition.

Identified with readability-redundant-declaration.