Implement -Wself-assign, which warns on code such as:

Implement -Wself-assign, which warns on code such as:

  int x = 42;
  x = x;  // Warns here.

The warning avoids macro expansions, templates, user-defined assignment
operators, and volatile types, so false positives are expected to be low.

The common (mis-)use of this code pattern is to silence unused variable
warnings, but a more idiomatic way of doing that is '(void)x;'.
A follow-up to this will add a note and fix-it hint suggesting this
replacement in cases where the StmtExpr consists precisely of the self
assignment.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@122804 91177308-0d34-0410-b5e6-96231b3b80d8