[Bug 2142542] [NEW] ROCM_PATH envvar excludes resource directory from device libs search path

[Cory Bloor]

Public bug reported:

# What was expected

clang should include the LLVM 21 resource directory in its search for
the ROCm device libs, regardless of whether the ROCM_PATH environment
variable is defined or not.

This should successfully build a program twice:

```
# apt install -y hipcc clang-21 rocm-device-libs-21
# cat > main.hip << END
int main() { return 0; }
END
# clang++-21 main.hip -lamdhip64 -o main
# ROCM_PATH=/usr clang++-21 main.hip -lamdhip64 -o main
```

# What happened instead

The program was only built successfully once:
```
# apt install -y hipcc clang-21 rocm-device-libs-21
# cat > main.hip << END
int main() { return 0; }
END
# clang++-21 main.hip -lamdhip64 -o main
# ROCM_PATH=/usr clang++-21 main.hip -lamdhip64 -o main
clang++-21: error: cannot find ROCm device library; provide its path via '--rocm-path' or '--rocm-device-lib-path', or pass '-nogpulib' to build without ROCm device library
```

# System Info

## lsb_release -rd
Description:	Ubuntu Resolute Raccoon (development branch)
Release:	26.04

## apt-cache policy llvm-21
llvm-21:
  Installed: 1:21.1.8-1ubuntu1
  Candidate: 1:21.1.8-1ubuntu1
  Version table:
 *** 1:21.1.8-1ubuntu1 500
        500 http://archive.ubuntu.com/ubuntu resolute/universe amd64 Packages
        100 /var/lib/dpkg/status

** Affects: llvm-toolchain-21 (Ubuntu)
     Importance: Undecided
         Status: New

** Affects: llvm-toolchain-21 (Debian)
     Importance: Unknown
         Status: Unknown

** Bug watch added: Debian Bug tracker #1125754
   https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1125754

** Also affects: llvm-toolchain-21 (Debian) via
   https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1125754
   Importance: Unknown
       Status: Unknown

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https://bugs.launchpad.net/bugs/2142542

Title:
  ROCM_PATH envvar excludes resource directory from device libs search
  path

Status in llvm-toolchain-21 package in Ubuntu:
  New
Status in llvm-toolchain-21 package in Debian:
  Unknown

Bug description:
  # What was expected

  clang should include the LLVM 21 resource directory in its search for
  the ROCm device libs, regardless of whether the ROCM_PATH environment
  variable is defined or not.

  This should successfully build a program twice:

  ```
  # apt install -y hipcc clang-21 rocm-device-libs-21
  # cat > main.hip << END
  int main() { return 0; }
  END
  # clang++-21 main.hip -lamdhip64 -o main
  # ROCM_PATH=/usr clang++-21 main.hip -lamdhip64 -o main
  ```

  # What happened instead

  The program was only built successfully once:
  ```
  # apt install -y hipcc clang-21 rocm-device-libs-21
  # cat > main.hip << END
  int main() { return 0; }
  END
  # clang++-21 main.hip -lamdhip64 -o main
  # ROCM_PATH=/usr clang++-21 main.hip -lamdhip64 -o main
  clang++-21: error: cannot find ROCm device library; provide its path via '--rocm-path' or '--rocm-device-lib-path', or pass '-nogpulib' to build without ROCm device library
  ```

  # System Info

  ## lsb_release -rd
  Description:	Ubuntu Resolute Raccoon (development branch)
  Release:	26.04

  ## apt-cache policy llvm-21
  llvm-21:
    Installed: 1:21.1.8-1ubuntu1
    Candidate: 1:21.1.8-1ubuntu1
    Version table:
   *** 1:21.1.8-1ubuntu1 500
          500 http://archive.ubuntu.com/ubuntu resolute/universe amd64 Packages
          100 /var/lib/dpkg/status

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