[analyzer] Add a checker option to detect nested dead stores

Enables the users to specify an optional flag which would warn for more dead
stores.
Previously it ignored if the dead store happened e.g. in an if condition.

if ((X = generate())) { // dead store to X
}

This patch introduces the `WarnForDeadNestedAssignments` option to the checker,
which is `false` by default - so this change would not affect any previous
users.
I have updated the code, tests and the docs as well. If I missed something, tell
me.

I also ran the analysis on Clang which generated 14 more reports compared to the
unmodified version. All of them seemed reasonable for me.

Related previous patches:
rGf224820b45c6847b91071da8d7ade59f373b96f3

Reviewers: NoQ, krememek, Szelethus, baloghadamsoftware

Reviewed By: Szelethus

Patch by Balázs Benics!

Differential Revision: https://reviews.llvm.org/D66733

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@370767 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 9 insertions, 0 deletions

diff --git a/docs/analyzer/checkers.rst b/docs/analyzer/checkers.rst
index f50fff918c..a751b14809 100644
--- a/docs/analyzer/checkers.rst
+++ b/docs/analyzer/checkers.rst
@@ -319,6 +319,15 @@ Check for values stored to variables that are never read afterwards.
    x = 1; // warn
  }
 
+The ``WarnForDeadNestedAssignments`` option enables the checker to emit
+warnings for nested dead assignments. You can disable with the
+``-analyzer-config deadcode.DeadStores:WarnForDeadNestedAssignments=false``.
+*Defaults to true*.
+
+Would warn for this e.g.:
+if ((y = make_int())) {
+}
+
 .. _nullability-checkers:
 
 nullability