[analyzer] Consolidate the va_list checkers (#156682)HEADmain

Previously the analyzer had an undocumented top-level checker group
called `valist` which offered several checkers to detect use of
uninitialized `va_list` objects and leaks of `va_list`s.

As the responsibilities of these checkers were messily intertwined and
`va_list` is a rarely used language feature, this commit simplifies the
situation by consolidating these checkers into a single checker which
will be called `security.VAList`.

Note that I'm choosing the capitalization `VAList` to be consistent with
the example of the AST node type `VAArgExpr`. I updated many variable
names to ensure that `ValistChecker.cpp` uses this spelling everywhere
(in CamelCased names). I'm planning to rename `ValistChecker.cpp` to
`VAListChecker.cpp` in a follow-up commit.

This commit also adds documentation for this checker in checkers.rst.

Among the test files I preserved the existing separation but I
eliminated some duplicated cases now that there is no way to separately
enable the old sub-checkers.

For the background of this change see also the discourse thread
https://discourse.llvm.org/t/clean-up-valist-checkers/85277/3

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