Add missing deps edge: CodeGenAction.cpp.o -> GenVT.inc (llvm#109306)

CodeGenAction.cpp.o depends on generating GenVT.inc before trying to
compile it through the following header chain:

```
GenVT.inc
MachineValueType.h
LowLevelType.h
MachineMemOperand.h
MachineInstr.h
MachineBasicBlock.h
MachineFunctionPass.h
MachineOptimizationRemarkEmitter.h
CodeGenAction.cpp
```

There is a dependency edge through LLVMCodeGenTypes, but that edge is
applied to the clangCodeGen link step, not the compile step of the files
that make up clangCodeGen. Usually the compile and link are close enough
in the build that GenVT.inc is scheduled early enough that it exists by
the time we're compiling CodeGenAction.cpp.o, but on machines with high
core counts, it seems to be more prevalent that the scheduling works out
just right to expose the missing edge. I've only been able to reproduce
this on machines with at least 64 cores (but even then it was not
reliable).

Additionally, llvm-tblgen depends on GenVT.inc, so to see the missing
dependency edge, one must be using a pre-built tablegen binary.

Adding the missing dependency edge to ensure that GenVT.inc is generated
before trying to compile CodeGenAction.cpp.o.

Found by inspecting the dependency graph generated from Ninja with:

```sh
cmake -G 'Ninja' \
  ...
  -DLLVM_TABLEGEN=<path to native tblegen> \
  -DCLANG_TABLEGEN=<path to native clang tblgen> \
  -DLLVM_ENABLE_PROJECTS=clang \
  ...

ninja -t graph \
  tools/clang/lib/CodeGen/CMakeFiles/obj.clangCodeGen.dir/CodeGenAction.cpp.o | dot -Tpdf > CodeGenAction.pdf
```

Author: etcwilde

clang/lib/CodeGen/CMakeLists.txt

@@ -144,6 +144,7 @@ add_clang_library(clangCodeGen
   VarBypassDetector.cpp
 
   DEPENDS
+  vt_gen
   intrinsics_gen
   ClangDriverOptions
   # These generated headers are included transitively.