[analyzer; new edges] In for(;;), use the ForStmt itself for loop notes.

Most loop notes (like "entering loop body") are attached to the condition
expression guarding a loop or its equivalent. For loops may not have a
condition expression, though. Rather than crashing, just use the entire
ForStmt as the location. This is probably the best we can do.

<rdar://problem/14016063>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@182904 91177308-0d34-0410-b5e6-96231b3b80d8