CStringChecker has a sub-checker alpha.unix.cstring.NotNullTerminated
which checks for invalid objects passed to string functions. The checker
and its name are not exact and more functions could be checked, this
change only adds some tests and improves documentation.

`CE->getCalleeDecl()` returns `VarDecl` if the callee is actually a
function pointer variable. Consequently, calling `getAsFunction()` will
return null.

To workaround the case, we should use the `CallEvent::parameters()`,
which will internally recover the function being called and do the right
thing.

Fixes #74269
Depends on "[analyzer][NFC] Prefer CallEvent over CallExpr in APIs"

Differential Revision: https://reviews.llvm.org/D55873

rdar://problem/45366551

llvm-svn: 349683