[Fixed Point Arithmetic] FixedPointCast

This patch is a part of https://reviews.llvm.org/D48456 in an attempt to
split them up. This contains the code for casting between fixed point types
and other fixed point types.

The method for converting between fixed point types is based off the convert()
method in APFixedPoint.

Differential Revision: https://reviews.llvm.org/D50616

llvm-svn: 344530
diff --git a/clang/lib/CodeGen/CGExprConstant.cpp b/clang/lib/CodeGen/CGExprConstant.cpp
index 651b05a..3dbb20d 100644
--- a/clang/lib/CodeGen/CGExprConstant.cpp
+++ b/clang/lib/CodeGen/CGExprConstant.cpp
@@ -871,6 +871,7 @@
     case CK_FloatingCast:
     case CK_ZeroToOCLEvent:
     case CK_ZeroToOCLQueue:
+    case CK_FixedPointCast:
       return nullptr;
     }
     llvm_unreachable("Invalid CastKind");