[Core] Grab-bag of improvements for Scalar.

Remove Scalar::Cast.

It was noted on the list that this method is unused. So, this patch
removes it.

Fix Scalar::Promote for most integer types

This fixes promotion of most integer types (128- and 256-bit types are
handled in a subsequent patch) to floating-point types. Previously
promotion was done bitwise, where value preservation is correct.

Fix Scalar::Promote for 128- and 256-bit integer types

This patch fixes the behavior of Scalar::Promote when trying to
perform a binary operation involving a 128- or 256-bit integer type
and a floating-point type. Now, the integer is cast to the floating
point type for the operation.

Patch by Tom Tromey!

Differential Revision:  https://reviews.llvm.org/D44907

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@328985 91177308-0d34-0410-b5e6-96231b3b80d8

Author: dcci

include/lldb/Core/Scalar.h

@@ -50,13 +50,13 @@ class Scalar {
     e_ulong,
     e_slonglong,
     e_ulonglong,
-    e_float,
-    e_double,
-    e_long_double,
-    e_uint128,
     e_sint128,
+    e_uint128,
+    e_sint256,
     e_uint256,
-    e_sint256
+    e_float,
+    e_double,
+    e_long_double
   };
 
   //------------------------------------------------------------------
@@ -165,8 +165,6 @@ class Scalar {
 
   bool Promote(Scalar::Type type);
 
-  bool Cast(Scalar::Type type);
-
   bool MakeSigned();
 
   bool MakeUnsigned();