[llvm] Reapply "[llvm-jitlink] Use concurrent linking by default." with fixes. (PR #120958)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-debian-cpp20` running on `clang-debian-cpp20` while building `llvm` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/108/builds/7490

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

```
Step 6 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'LLVM :: ExecutionEngine/JITLink/x86-64/LocalDependencyPropagation.s' FAILED ********************
Exit Code: 1

Command Output (stderr):
--
RUN: at line 2: /vol/worker/clang-debian-cpp20/clang-debian-cpp20/build/bin/llvm-mc -triple=x86_64-apple-macosx10.9 -filetype=obj -o /vol/worker/clang-debian-cpp20/clang-debian-cpp20/build/test/ExecutionEngine/JITLink/x86-64/Output/LocalDependencyPropagation.s.tmp /vol/worker/clang-debian-cpp20/clang-debian-cpp20/llvm-project/llvm/test/ExecutionEngine/JITLink/x86-64/LocalDependencyPropagation.s
+ /vol/worker/clang-debian-cpp20/clang-debian-cpp20/build/bin/llvm-mc -triple=x86_64-apple-macosx10.9 -filetype=obj -o /vol/worker/clang-debian-cpp20/clang-debian-cpp20/build/test/ExecutionEngine/JITLink/x86-64/Output/LocalDependencyPropagation.s.tmp /vol/worker/clang-debian-cpp20/clang-debian-cpp20/llvm-project/llvm/test/ExecutionEngine/JITLink/x86-64/LocalDependencyPropagation.s
RUN: at line 3: /vol/worker/clang-debian-cpp20/clang-debian-cpp20/build/bin/llvm-jitlink -debug-only=orc -noexec -abs _external_func=0x1    -entry=_foo /vol/worker/clang-debian-cpp20/clang-debian-cpp20/build/test/ExecutionEngine/JITLink/x86-64/Output/LocalDependencyPropagation.s.tmp 2>&1 | /vol/worker/clang-debian-cpp20/clang-debian-cpp20/build/bin/FileCheck /vol/worker/clang-debian-cpp20/clang-debian-cpp20/llvm-project/llvm/test/ExecutionEngine/JITLink/x86-64/LocalDependencyPropagation.s
+ /vol/worker/clang-debian-cpp20/clang-debian-cpp20/build/bin/FileCheck /vol/worker/clang-debian-cpp20/clang-debian-cpp20/llvm-project/llvm/test/ExecutionEngine/JITLink/x86-64/LocalDependencyPropagation.s
+ /vol/worker/clang-debian-cpp20/clang-debian-cpp20/build/bin/llvm-jitlink -debug-only=orc -noexec -abs _external_func=0x1 -entry=_foo /vol/worker/clang-debian-cpp20/clang-debian-cpp20/build/test/ExecutionEngine/JITLink/x86-64/Output/LocalDependencyPropagation.s.tmp
/vol/worker/clang-debian-cpp20/clang-debian-cpp20/llvm-project/llvm/test/ExecutionEngine/JITLink/x86-64/LocalDependencyPropagation.s:17:10: error: CHECK: expected string not found in input
# CHECK: Simplified dependencies:
         ^
<stdin>:85:65: note: scanning from here
 Symbols: { _foo }, Dependencies: { (main, { _external_func }) }
                                                                ^
<stdin>:93:13: note: possible intended match here
Destroying JITDylib Process
            ^

Input file: <stdin>
Check file: /vol/worker/clang-debian-cpp20/clang-debian-cpp20/llvm-project/llvm/test/ExecutionEngine/JITLink/x86-64/LocalDependencyPropagation.s

-dump-input=help explains the following input dump.

Input was:
<<<<<<
            .
            .
            .
           80:  Symbols: { }, Dependencies: { } 
           81:  _foo }, Dependencies: { (main, { _external_func }) } 
           82:  Symbols: { _baz }, Dependencies: { (main, { _foo }) } 
           83:  Simplified dependencies: 
           84:  Symbols: { _baz }, Dependencies: { (main, { _external_func }) } 
           85:  Symbols: { _foo }, Dependencies: { (main, { _external_func }) } 
check:17'0                                                                     X error: no match found
           86: Dispatching: Execute query complete callback for { ("_foo": 0x7c4d80001000 [Callable]) } 
check:17'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
           87: Ending ExecutionSession 0x5b6b42e42da0 
check:17'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
           88: In main removing tracker 0x5b6b42e43940 
check:17'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
           89: In main destroying tracker 0x5b6b42e43940 
check:17'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
           90: In Process removing tracker 0x5b6b42e46410 
check:17'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
           91: In Process destroying tracker 0x5b6b42e46410 
check:17'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
           92: Destroying JITDylib main 
...

```

</details>

https://github.com/llvm/llvm-project/pull/120958