[SourceKit] Check location validity before getting the line number to fix a crash. rdar://26756189 (#3364)

Author: nkcsgexi

lib/IDE/Formatting.cpp

@@ -477,7 +477,7 @@ class FormatWalker : public SourceEntityWalker {
         return;
       SourceLoc PrevLoc;
       auto FindAlignLoc = [&](SourceLoc Loc) {
-        if (PrevLoc.isValid() &&
+        if (PrevLoc.isValid() && Loc.isValid() &&
             SM.getLineNumber(PrevLoc) == SM.getLineNumber(Loc))
           return PrevLoc;
         return PrevLoc = Loc;

test/SourceKit/CodeFormat/indent-crashes.swift

@@ -0,0 +1,10 @@
+subscript (__PLACEHOLDER_0__) -> __PLACEHOLDER_1__ {
+  get {
+     __PLACEHOLDER_2__
+  }
+  set(__PLACEHOLDER_3__) {
+     __PLACEHOLDER_4__
+  }
+}
+
+// RUN: %sourcekitd-test -req=format -line=1 -length=1 %s >%t.response