[clang] [clang] Increase VecLib bitfield size to 4 bits in CodeGenOptions.def (PR #108804)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

I had to revert due to build breakages (linked in comments):
```
CMake Error at cmake/modules/LLVMProcessSources.cmake:116 (message):
  Found erroneous configuration for source file SimulatedOverflowTest.cpp
  LLVM's build system enforces that all source files are added to a build
  target, that exactly one build target exists in each directory, and that
  this target lists all files in that directory.  If you want multiple
  targets in the same directory, add PARTIAL_SOURCES_INTENDED to the target
  specification, though it is discouraged.
  Please update
  C:/buildbot/as-builder-3/llvm-clang-x86_64-win-fast/llvm-project/clang/unittests/CodeGen/CMakeLists.txt
```

I didn't notice that you were missing changes to CMakeLists.txt to add the new files.

https://github.com/llvm/llvm-project/pull/108804