[ConstraintLocator] Add a isFirstElement helper function

Author: hborla

lib/Sema/ConstraintLocator.cpp

@@ -249,8 +249,7 @@ bool ConstraintLocator::isForOptionalTry() const {
 }
 
 bool ConstraintLocator::isForFunctionBuilderBodyResult() const {
-  auto elt = getFirstElementAs<LocatorPathElt::FunctionBuilderBodyResult>();
-  return elt.hasValue();
+  return isFirstElement<LocatorPathElt::FunctionBuilderBodyResult>();
 }
 
 GenericTypeParamType *ConstraintLocator::getGenericParameter() const {

lib/Sema/ConstraintLocator.h

@@ -384,6 +384,14 @@ class ConstraintLocator : public llvm::FoldingSetNode {
     return anchor && isa<E>(anchor) && getPath().empty();
   }
 
+  /// Check whether the first element in the path of this locator (if any)
+  /// is a given \c LocatorPathElt subclass.
+  template <class T>
+  bool isFirstElement() const {
+    auto path = getPath();
+    return !path.empty() && path.front().is<T>();
+  }
+
   /// Attempts to cast the first path element of the locator to a specific
   /// \c LocatorPathElt subclass, returning \c None if either unsuccessful or
   /// the locator has no path elements.