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
```
This reverts commit 54db16221c92eb52efbea90ad5b5d2a1d00cda3e.
Check for existence of __SIZOEF_INT128__ so we don't run those
tests on targets that don't have int128.
