[llvm] [TableGen][CodeEmitterGen] Cache Target/CGH in class (NFC) (PR #158517)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `ppc64le-lld-multistage-test` running on `ppc64le-lld-multistage-test` while building `llvm` at step 12 "build-stage2-unified-tree".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/168/builds/16154

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 12 (build-stage2-unified-tree) failure: build (failure) (timed out)
...
950.817 [57/9/6629] Linking CXX shared library lib/libLTO.so.22.0git
950.910 [56/9/6630] Linking CXX executable unittests/Target/TargetMachineCTests
950.910 [56/8/6631] Creating library symlink lib/libLTO.so
950.976 [56/7/6632] Linking CXX executable bin/clang-sycl-linker
951.007 [56/6/6633] Linking CXX executable bin/dsymutil
951.138 [56/5/6634] Linking CXX executable bin/opt
951.249 [56/4/6635] Linking CXX executable bin/llvm-lto
951.443 [56/3/6636] Linking CXX executable unittests/CodeGen/CGPluginTest/CGPluginTest
972.969 [56/2/6637] Building CXX object tools/bugpoint-passes/CMakeFiles/BugpointPasses.dir/TestPasses.cpp.o
974.220 [55/2/6638] Linking CXX shared module lib/BugpointPasses.so
command timed out: 1200 seconds without output running [b'ninja'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=2174.739730

```

</details>

https://github.com/llvm/llvm-project/pull/158517