Ease cherry-picking by providing an impl of SILType::getASTType().

SILType::getSwiftRValueType() was renamed to SILType::getASTType() on
master. This commit provides SILType::getASTType() and delegates to
getSwiftRValueType() to ease cherry-picking from master -> 4.2.

Otherwise, all of the times someone on master cherry-picks such code, we will
get a compilation error.

Author: gottesmm

include/swift/SIL/SILType.h

@@ -158,6 +158,11 @@ class SILType {
     return SILType(getSwiftRValueType(), SILValueCategory::Object);
   }
 
+  /// Returns the canonical AST type referenced by this SIL type.
+  CanType getASTType() const {
+    return getSwiftRValueType();
+  }
+
   /// Returns the Swift type referenced by this SIL type.
   CanType getSwiftRValueType() const {
     return CanType(value.getPointer());