[Cocoa] Prefer llvm::SignExtend64. Pointed out by zturner.

llvm-svn: 347087
diff --git a/lldb/source/Plugins/Language/ObjC/Cocoa.cpp b/lldb/source/Plugins/Language/ObjC/Cocoa.cpp
index 4aa3e0b..d34b78b 100644
--- a/lldb/source/Plugins/Language/ObjC/Cocoa.cpp
+++ b/lldb/source/Plugins/Language/ObjC/Cocoa.cpp
@@ -773,8 +773,7 @@
   int64_t exp7 = exp;
   // Tagged exponent field is 7-bit signed. Sign-extend the value to 64 bits
   // before performing arithmetic.
-  int64_t exp11 = ((exp7 << 57) >> 57) + TAGGED_DATE_EXPONENT_BIAS;
-  return exp11;
+  return llvm::SignExtend64<7>(exp) + TAGGED_DATE_EXPONENT_BIAS;
 }
 
 static uint64_t decodeTaggedTimeInterval(uint64_t encodedTimeInterval) {