[llvm] [NFC][TableGen] Move decoder tests to DecoderEmitter directory (PR #159040)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-aarch64-quick` running on `linaro-clang-aarch64-quick` while building `llvm` at step 5 "ninja check 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/65/builds/22701

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

```
Step 5 (ninja check 1) failure: stage 1 checked (failure)
******************** TEST 'Clangd Unit Tests :: ./ClangdTests/163/332' FAILED ********************
Script(shard):
--
GTEST_OUTPUT=json:/home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/tools/clang/tools/extra/clangd/unittests/./ClangdTests-Clangd Unit Tests-1656257-163-332.json GTEST_SHUFFLE=0 GTEST_TOTAL_SHARDS=332 GTEST_SHARD_INDEX=163 /home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/tools/clang/tools/extra/clangd/unittests/./ClangdTests
--

Note: This is test shard 164 of 332.
[==========] Running 4 tests from 4 test suites.
[----------] Global test environment set-up.
[----------] 1 test from CompletionTest
[ RUN      ] CompletionTest.DeprecatedResults
Built preamble of size 707604 for file /clangd-test/foo.cpp version null in 0.16 seconds
Code complete: sema context Statement, query scopes [] (AnyScope=false), expected type <none>
Code complete: 2 results from Sema, 0 from Index, 0 matched, 0 from identifiers, 2 returned.
[       OK ] CompletionTest.DeprecatedResults (188 ms)
[----------] 1 test from CompletionTest (188 ms total)

[----------] 1 test from FileIndexTest
[ RUN      ] FileIndexTest.ReferencesInMainFileWithPreamble
Built preamble of size 710960 for file /clangd-test/TestTU.cpp version null in 0.16 seconds
indexed file AST for /clangd-test/TestTU.cpp version null:
  symbol slab: 1 symbols, 4448 bytes
  ref slab: 2 symbols, 2 refs, 4272 bytes
  relations slab: 0 relations, 24 bytes
Build dynamic index for main-file symbols with estimated memory usage of 11576 bytes
Built preamble of size 710960 for file /clangd-test/TestTU.cpp version null in 0.11 seconds
indexed preamble AST for /clangd-test/TestTU.cpp version null:
  symbol slab: 2 symbols, 4680 bytes
  ref slab: 0 symbols, 0 refs, 128 bytes
  relations slab: 0 relations, 24 bytes
[       OK ] FileIndexTest.ReferencesInMainFileWithPreamble (328 ms)
[----------] 1 test from FileIndexTest (329 ms total)

[----------] 1 test from JSONTransportTest
[ RUN      ] JSONTransportTest.EndOfFile
<<< {"jsonrpc":"2.0","method":"call","params":1234}

>>> {"id":42,"jsonrpc":"2.0","method":"echo call","params":1234}

[       OK ] JSONTransportTest.EndOfFile (0 ms)
[----------] 1 test from JSONTransportTest (0 ms total)

[----------] 1 test from GoToInclude
[ RUN      ] GoToInclude.All
ASTWorker building file /clangd-test/foo.h version null with command 
[/clangd-test]
clang -ffreestanding /clangd-test/foo.h
Driver produced command: cc1 -cc1 -triple aarch64-unknown-linux-gnu -fsyntax-only -disable-free -clear-ast-before-backend -main-file-name foo.h -mrelocation-model pic -pic-level 2 -pic-is-pie -mframe-pointer=non-leaf -fmath-errno -ffp-contract=on -fno-rounding-math -mconstructor-aliases -ffreestanding -enable-tlsdesc -target-cpu generic -target-feature +v8a -target-feature +fp-armv8 -target-feature +neon -target-abi aapcs -debugger-tuning=gdb -fdebug-compilation-dir=/clangd-test -fcoverage-compilation-dir=/clangd-test -resource-dir lib/clang/22 -internal-isystem lib/clang/22/include -internal-isystem /usr/local/include -internal-externc-isystem /include -internal-externc-isystem /usr/include -ferror-limit 19 -fno-signed-char -fgnuc-version=4.2.1 -fskip-odr-check-in-gmf -no-round-trip-args -target-feature -fmv -faddrsig -x c-header /clangd-test/foo.h
Building first preamble for /clangd-test/foo.h version null
Built preamble of size 730288 for file /clangd-test/foo.h version null in 1.29 seconds
...

```

</details>

https://github.com/llvm/llvm-project/pull/159040