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.

llvm-svn: 122804