AST: avoid unnecessarily calling getStartLoc() to avoid exponential growth of the stack trace

Issuing multiple getStartLoc() from sub-expression can exponentially grow the stack trace.
When the expression under analysis is complex enough, this could be a user-noticeable hang.
This patch fixes UnresolvedDotExpr::getStartLoc() by 'refactoring' the result of SubExpr->getStartLoc()
to a local variable.

rdar://52982457

Author: nkcsgexi

include/swift/AST/Expr.h

@@ -2387,8 +2387,9 @@ class UnresolvedDotExpr : public Expr {
   }
 
   SourceLoc getStartLoc() const {
-    if (SubExpr->getStartLoc().isValid())
-      return SubExpr->getStartLoc();
+    auto SubLoc = SubExpr->getStartLoc();
+    if (SubLoc.isValid())
+      return SubLoc;
     else if (DotLoc.isValid())
       return DotLoc;
     else

test/IDE/coloring_52982457.swift

@@ -0,0 +1,115 @@
+// RUN: %target-swift-ide-test -syntax-coloring -source-filename %s | %FileCheck %s
+// RUN: %target-swift-ide-test -syntax-coloring -typecheck -source-filename %s | %FileCheck %s
+
+func migrate() -> EitherIO<Error, Prelude.Unit> {
+// CHECK: <kw>func</kw> migrate() -> <type>EitherIO</type><<type>Error</type>, <type>Prelude</type>.<type>Unit</type>> {
+  return self.execute(
+    """
+  """
+    )
+    .flatMap(const(execute(
+      """
+    """
+    )))
+    .flatMap(const(execute(
+      """
+    """
+    )))
+    .flatMap(const(execute(
+      """
+    """
+    )))
+    .flatMap(const(execute(
+      """
+    """
+    )))
+    .flatMap(const(execute(
+      """
+    """
+    )))
+    .flatMap(const(execute(
+      """
+    """
+    )))
+    .flatMap(const(execute(
+      """
+    """
+    )))
+    .flatMap(const(execute(
+      """
+    """
+    )))
+    .flatMap(const(execute(
+      """
+    """
+    )))
+    .flatMap(const(execute(
+      """
+    """
+    )))
+    .flatMap(const(execute(
+      """
+    """
+    )))
+    .flatMap(const(execute(
+      """
+    """
+    )))
+    .flatMap(const(execute(
+      """
+    """
+    )))
+    .flatMap(const(execute(
+      """
+    """
+    )))
+    .flatMap(const(execute(
+      """
+    """
+    )))
+    .flatMap(const(execute(
+      """
+    """
+    )))
+    .flatMap(const(execute(
+      """
+    """
+    )))
+    .flatMap(const(execute(
+      """
+    """
+    )))
+    .flatMap(const(execute(
+      """
+    """
+    )))
+    .flatMap(const(execute(
+      """
+    """
+    )))
+    .flatMap(const(execute(
+      """
+    """
+    )))
+    .flatMap(const(execute(
+      """
+    """
+    )))
+    .flatMap(const(execute(
+      """
+    """
+    )))
+    .flatMap(const(execute(
+      """
+    """
+    )))
+    .flatMap(const(execute(
+      """
+    """
+    )))
+    .flatMap(const(execute(
+      """
+    """
+    )))
+    .map(const(unit))
+}