[clang][unittests] On Windows, silence warning when building with MSVC

Fixes:
```
[113/324] Building CXX object tools\clang\unittests\AST\ByteCode\CMakeFiles\InterpTests.dir\BitcastBuffer.cpp.obj
C:\git\llvm-project\clang\unittests\AST\ByteCode\BitcastBuffer.cpp(52): warning C4309: 'initializing': truncation of constant value
C:\git\llvm-project\clang\unittests\AST\ByteCode\BitcastBuffer.cpp(53): warning C4309: 'initializing': truncation of constant value
```

Author: aganea

clang/unittests/AST/ByteCode/BitcastBuffer.cpp

@@ -49,8 +49,8 @@ TEST(BitcastBuffer, PushData) {
 
   BitcastBuffer Buff2(Bytes(sizeof(int)).toBits());
   {
-    short s1 = 0xCAFE;
-    short s2 = 0xBABE;
+    short s1 = (short)0xCAFE;
+    short s2 = (short)0xBABE;
     std::byte sdata[2];
 
     std::memcpy(sdata, &s1, sizeof(s1));