[llvm] Add a pass to collect dropped var stats for MIR (PR #120501)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-cmake-x86_64-avx512-linux` running on `avx512-intel64` while building `llvm` at step 7 "ninja check 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/133/builds/8668

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

```
Step 7 (ninja check 1) failure: stage 1 checked (failure)
******************** TEST 'LLVM-Unit :: MIR/./MIRTests/0/17' FAILED ********************
Script(shard):
--
GTEST_OUTPUT=json:/localdisk2/buildbot/llvm-worker/clang-cmake-x86_64-avx512-linux/stage1/unittests/MIR/./MIRTests-LLVM-Unit-2960713-0-17.json GTEST_SHUFFLE=0 GTEST_TOTAL_SHARDS=17 GTEST_SHARD_INDEX=0 /localdisk2/buildbot/llvm-worker/clang-cmake-x86_64-avx512-linux/stage1/unittests/MIR/./MIRTests
--

Note: This is test shard 1 of 17.
[==========] Running 1 test from 1 test suite.
[----------] Global test environment set-up.
[----------] 1 test from DroppedVariableStatsMIR
[ RUN      ] DroppedVariableStatsMIR.BothDeleted
Stack dump without symbol names (ensure you have llvm-symbolizer in your PATH or set the environment var `LLVM_SYMBOLIZER_PATH` to point to it):
0  MIRTests  0x000000000140f711
1  MIRTests  0x000000000140cf24
2  libc.so.6 0x00007f2f4f801db0
3  MIRTests  0x0000000000bf417b
4  MIRTests  0x000000000046f3e9
5  MIRTests  0x0000000001430b93
6  MIRTests  0x000000000143ae92
7  MIRTests  0x000000000143b428
8  MIRTests  0x000000000144538a
9  MIRTests  0x00000000014300ab
10 MIRTests  0x0000000000458866
11 libc.so.6 0x00007f2f4f7ece50
12 libc.so.6 0x00007f2f4f7ecefc __libc_start_main + 124
13 MIRTests  0x000000000046d215

--
exit: -11
--
shard JSON output does not exist: /localdisk2/buildbot/llvm-worker/clang-cmake-x86_64-avx512-linux/stage1/unittests/MIR/./MIRTests-LLVM-Unit-2960713-0-17.json
********************


```

</details>

https://github.com/llvm/llvm-project/pull/120501