[llvm] [llvm] Enable LLVM_LINK_LLVM_DYLIB by default on non-Windows platforms (PR #138187)

[Reid Kleckner via llvm-commits]

rnk wrote:

The premerge check failures seem related, and they seem related to bolt:
```
llvm-lit: /usr/local/google/home/rnk/llvm-project/llvm/utils/lit/lit/formats/googletest.py:39: warning: unable to discover google-tests in '/usr/local/google/home/rnk/llvm-project/build_cmake/tools/bolt/unittests/Core/./CoreTests': Command '['/usr/local/google/home/rnk/llvm-project/build_cmake/tools/bolt/unittests/Core/./CoreTests', '--gtest_list_tests', '--gtest_filter=-*DISABLED_*']' died with <Signals.SIGABRT: 6>.. Process output: b''
LLVM ERROR: Option 'si' already exists!
llvm-lit: /usr/local/google/home/rnk/llvm-project/llvm/utils/lit/lit/formats/googletest.py:39: warning: unable to discover google-tests in '/usr/local/google/home/rnk/llvm-project/build_cmake/tools/bolt/unittests/Profile/./ProfileTests': Command '['/usr/local/google/home/rnk/llvm-project/build_cmake/tools/bolt/unittests/Profile/./ProfileTests', '--gtest_list_tests', '--gtest_filter=-*DISABLED_*']' died with <Signals.SIGABRT: 6>.. Process output: b''
FAIL: BOLT-Unit :: Profile/./ProfileTests/failed_to_discover_tests_from_gtest (1 of 2)
******************** TEST 'BOLT-Unit :: Profile/./ProfileTests/failed_to_discover_tests_from_gtest' FAILED ********************

********************
FAIL: BOLT-Unit :: Core/./CoreTests/failed_to_discover_tests_from_gtest (2 of 2)
******************** TEST 'BOLT-Unit :: Core/./CoreTests/failed_to_discover_tests_from_gtest' FAILED ********************

********************
********************
Failed Tests (2):
  BOLT-Unit :: Core/./CoreTests/failed_to_discover_tests_from_gtest
  BOLT-Unit :: Profile/./ProfileTests/failed_to_discover_tests_from_gtest


Testing Time: 0.02s

Total Discovered Tests: 2
  Failed: 2 (100.00%)
```

It was pointed out in the RFC that Bolt links LLVM statically instead of using the dylib because it directly refers to symbols in Target libraries that are hidden visibility. I think resolving that may become a blocker.

https://github.com/llvm/llvm-project/pull/138187