Formatting: Avoid crashing when indent width is 0. rdar://37835956 (#14873)

Author: nkcsgexi

lib/IDE/Formatting.cpp

@@ -843,6 +843,9 @@ class CodeFormatter {
     auto AddIndentFunc = [&] () {
       auto Width = FmtOptions.UseTabs ? FmtOptions.TabWidth
                                       : FmtOptions.IndentWidth;
+      // We don't need to add additional indentation if Width is zero.
+      if (!Width)
+        return;
       // Increment indent.
       ExpandedIndent += Width;
       // Normalize indent to align on proper column indent width.

test/SourceKit/CodeFormat/indent-zero-width.swift

@@ -0,0 +1,22 @@
+class Foo {
+
+var test : Int
+
+	func foo() {
+		test = 1
+	}
+
+}
+func bar(a: Int,
+b: Int) {}
+
+// RUN: %sourcekitd-test -req=format -line=1 -length=1 -req-opts=usetabs=0 -req-opts=indentwidth=0 %s >%t.response
+// RUN: %sourcekitd-test -req=format -line=3 -length=1 -req-opts=usetabs=0 -req-opts=indentwidth=0 %s >>%t.response
+// RUN: %sourcekitd-test -req=format -line=6 -length=1 -req-opts=usetabs=0 -req-opts=indentwidth=0 %s >>%t.response
+// RUN: %sourcekitd-test -req=format -line=11 -length=1 -req-opts=usetabs=0 -req-opts=indentwidth=0 %s >>%t.response
+// RUN: %FileCheck --strict-whitespace %s <%t.response
+
+// CHECK: key.sourcetext: "class Foo {"
+// CHECK: key.sourcetext: "var test : Int"
+// CHECK: key.sourcetext: "    test = 1"
+// CHECK: key.sourcetext: "         b: Int) {}"