Add support for clang with OpenMP and other minor changes to oneapi build system (#42717)

* Add support for clang in oneapi packages with OpenMP

* Add fallback search for libomp in OneApi package with OpenMP threading

* Add requires for the compiler when using threads=openmp in intel-oneapi-mkl

* Cosmetic changes to messages in oneapi.py

* Update error message in oneapi.py

Co-authored-by: Robert Cohn <rscohn2@gmail.com>

* Update another error message in oneapi.py

Co-authored-by: Robert Cohn <rscohn2@gmail.com>

* Inline helper error function in oneapi.py

* Update one more error message in oneapi.py

* Wrap long line in oneapi.py

---------

Co-authored-by: Robert Cohn <rscohn2@gmail.com>

lib/spack/spack/build_systems/oneapi.py

@@ -22,13 +22,6 @@
 from .generic import Package
 
 
-def raise_lib_error(*args):
-    """Bails out with an error message. Shows args after the first as one per
-    line, tab-indented, useful for long paths to line up and stand out.
-    """
-    raise InstallError("\n\t".join(str(i) for i in args))
-
-
 class IntelOneApiPackage(Package):
     """Base class for Intel oneAPI packages."""
 
@@ -206,20 +199,35 @@ def openmp_libs(self):
             libname = "libiomp5"
         elif self.spec.satisfies("%gcc"):
             libname = "libgomp"
+        elif self.spec.satisfies("%clang"):
+            libname = "libomp"
         else:
-            raise_lib_error("MKL OMP requires one of %gcc, %oneapi,or %intel")
+            raise InstallError(
+                "OneAPI package with OpenMP threading requires one of %clang, %gcc, %oneapi, "
+                "or %intel"
+            )
 
         # query the compiler for the library path
         with self.compiler.compiler_environment():
             omp_lib_path = Executable(self.compiler.cc)(
                 "--print-file-name", f"{libname}.{dso_suffix}", output=str
             ).strip()
+
+        # Newer versions of clang do not give the full path to libomp. If that's
+        # the case, look in a path relative to the compiler where libomp is
+        # typically found. If it's not found there, error out.
+        if not os.path.exists(omp_lib_path) and self.spec.satisfies("%clang"):
+            compiler_root = os.path.dirname(os.path.dirname(os.path.realpath(self.compiler.cc)))
+            omp_lib_path_compiler = os.path.join(compiler_root, "lib", f"{libname}.{dso_suffix}")
+            if os.path.exists(omp_lib_path_compiler):
+                omp_lib_path = omp_lib_path_compiler
+
         # if the compiler cannot find the file, it returns the input path
         if not os.path.exists(omp_lib_path):
-            raise_lib_error(f"Cannot locate OpenMP library: {omp_lib_path}")
+            raise InstallError(f"OneAPI package cannot locate OpenMP library: {omp_lib_path}")
 
         omp_libs = LibraryList(omp_lib_path)
-        tty.info(f"mkl requires openmp library: {omp_libs}")
+        tty.info(f"OneAPI package requires OpenMP library: {omp_libs}")
         return omp_libs
 
     # find_headers uses heuristics to determine the include directory

var/spack/repos/builtin/packages/intel-oneapi-mkl/package.py

@@ -125,6 +125,16 @@ class IntelOneapiMkl(IntelOneApiLibraryPackage):
         multi=False,
     )
 
+    requires(
+        "%clang",
+        "%gcc",
+        "%intel",
+        "%oneapi",
+        policy="one_of",
+        when="threads=openmp",
+        msg="MKL with OpenMP threading requires GCC, clang, or Intel compilers",
+    )
+
     depends_on("tbb")
     # cluster libraries need mpi
     depends_on("mpi", when="+cluster")