C++: Update dbscheme and add upgrade and downgrade scripts

cpp/downgrades/d77c09d8bdc172c9201dec293de1e14c931d3f05/upgrade.properties

@@ -0,0 +1,2 @@
+description: Remove _Float128 type
+compatibility: full

cpp/ql/lib/semmlecode.cpp.dbscheme

@@ -608,7 +608,7 @@ case @builtintype.kind of
 | 47 = @std_float64           // _Float64
 | 48 = @float64x              // _Float64x
 | 49 = @std_float128          // _Float128
-| 50 = @float128x             // _Float128x
+// ... 50 _Float128x
 | 51 = @char8_t
 | 52 = @float16               // _Float16
 | 53 = @complex_float16       // _Complex _Float16

cpp/ql/lib/upgrades/19887dbd33327fb07d54251786e0cb2578539775/builtintypes.ql

@@ -0,0 +1,13 @@
+class BuiltinType extends @builtintype {
+  string toString() { none() }
+}
+
+predicate isFloat128xBuiltinType(BuiltinType type) {
+  exists(int kind | builtintypes(type, _, kind, _, _, _) | kind = 50)
+}
+
+from BuiltinType type, string name, int kind, int kind_new, int size, int sign, int alignment
+where
+  builtintypes(type, name, kind, size, sign, alignment) and
+  if isFloat128xBuiltinType(type) then kind_new = 1 else kind_new = kind
+select type, name, kind_new, size, sign, alignment

cpp/ql/lib/upgrades/19887dbd33327fb07d54251786e0cb2578539775/upgrade.properties

@@ -0,0 +1,3 @@
+description: Remove _Float128 type
+compatibility: partial
+builtintypes.rel: run builtintypes.qlo