[clang] [flang] [mlir] [flang] Add support for -mprefer-vector-width=<value> (PR #142073)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `ppc64le-flang-rhel-clang` running on `ppc64le-flang-rhel-test` while building `clang,flang,mlir` at step 6 "test-build-unified-tree-check-flang".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/157/builds/29494

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

```
Step 6 (test-build-unified-tree-check-flang) failure: test (failure)
******************** TEST 'Flang :: Driver/prefer-vector-width.f90' FAILED ********************
Exit Code: 1

Command Output (stderr):
--
/home/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/build/bin/flang -fc1 -emit-llvm -o - /home/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/llvm-project/flang/test/Driver/prefer-vector-width.f90 2>&1| /home/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/build/bin/FileCheck /home/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/llvm-project/flang/test/Driver/prefer-vector-width.f90 --check-prefix=CHECK-DEF # RUN: at line 3
+ /home/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/build/bin/FileCheck /home/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/llvm-project/flang/test/Driver/prefer-vector-width.f90 --check-prefix=CHECK-DEF
+ /home/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/build/bin/flang -fc1 -emit-llvm -o - /home/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/llvm-project/flang/test/Driver/prefer-vector-width.f90
/home/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/build/bin/flang -fc1 -mprefer-vector-width=none -emit-llvm -o - /home/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/llvm-project/flang/test/Driver/prefer-vector-width.f90 2>&1| /home/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/build/bin/FileCheck /home/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/llvm-project/flang/test/Driver/prefer-vector-width.f90 --check-prefix=CHECK-NONE # RUN: at line 4
+ /home/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/build/bin/flang -fc1 -mprefer-vector-width=none -emit-llvm -o - /home/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/llvm-project/flang/test/Driver/prefer-vector-width.f90
+ /home/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/build/bin/FileCheck /home/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/llvm-project/flang/test/Driver/prefer-vector-width.f90 --check-prefix=CHECK-NONE
/home/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/llvm-project/flang/test/Driver/prefer-vector-width.f90:13:15: error: CHECK-NONE: expected string not found in input
! CHECK-NONE: attributes #0 = { "prefer-vector-width"="none" }
              ^
<stdin>:1:1: note: scanning from here
; ModuleID = 'FIRModule'
^
<stdin>:10:1: note: possible intended match here
attributes #0 = { "prefer-vector-width"="none" "target-features"="+64bit" }
^

Input file: <stdin>
Check file: /home/buildbots/llvm-external-buildbots/workers/ppc64le-flang-rhel-test/ppc64le-flang-rhel-clang-build/llvm-project/flang/test/Driver/prefer-vector-width.f90

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

Input was:
<<<<<<
            1: ; ModuleID = 'FIRModule' 
check:13'0     X~~~~~~~~~~~~~~~~~~~~~~~~ error: no match found
            2: source_filename = "FIRModule" 
check:13'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
            3: target datalayout = "e-m:e-Fn32-i64:64-i128:128-n32:64-S128-v256:256:256-v512:512:512" 
check:13'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
            4: target triple = "powerpc64le-unknown-linux-gnu" 
check:13'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
            5:  
check:13'0     ~
            6: define void @func_() #0 { 
check:13'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~
            7:  ret void 
check:13'0     ~~~~~~~~~~
            8: } 
check:13'0     ~~
            9:  
check:13'0     ~
           10: attributes #0 = { "prefer-vector-width"="none" "target-features"="+64bit" } 
check:13'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
check:13'1     ?                                                                            possible intended match
           11:  
...

```

</details>

https://github.com/llvm/llvm-project/pull/142073