clang/llvm: fix version detection (#19978)

This PR fixes two problems with clang/llvm's version detection. clang's
version output looks like this:

```
clang version 11.0.0
Target: x86_64-unknown-linux-gnu
```

This caused clang's version to be misdetected as:

```
clang@11.0.0
Target:
```

This resulted in errors when trying to actually use it as a compiler.

When using `spack external find`, we couldn't determine the compiler
version, resulting in errors like this:

```
==> Warning: "llvm@11.0.0+clang+lld+lldb" has been detected on the system but will not be added to packages.yaml [reason=c compiler not found for llvm@11.0.0+clang+lld+lldb]
```

Changing the regex to only match until the end of the line fixes these
problems.

Fixes: #19473

lib/spack/spack/compilers/clang.py

@@ -159,11 +159,11 @@ def extract_version_from_output(cls, output):
 
         match = re.search(
             # Normal clang compiler versions are left as-is
-            r'clang version ([^ )]+)-svn[~.\w\d-]*|'
+            r'clang version ([^ )\n]+)-svn[~.\w\d-]*|'
             # Don't include hyphenated patch numbers in the version
             # (see https://github.com/spack/spack/pull/14365 for details)
-            r'clang version ([^ )]+?)-[~.\w\d-]*|'
+            r'clang version ([^ )\n]+?)-[~.\w\d-]*|'
-            r'clang version ([^ )]+)',
+            r'clang version ([^ )\n]+)',
             output
         )
         if match:

lib/spack/spack/test/compilers/detection.py

@@ -96,7 +96,11 @@ def test_apple_clang_version_detection(
     ('clang version 8.0.0-3 (tags/RELEASE_800/final)\n'
      'Target: aarch64-unknown-linux-gnu\n'
      'Thread model: posix\n'
-     'InstalledDir: /usr/bin\n', '8.0.0')
+     'InstalledDir: /usr/bin\n', '8.0.0'),
+    ('clang version 11.0.0\n'
+     'Target: aarch64-unknown-linux-gnu\n'
+     'Thread model: posix\n'
+     'InstalledDir: /usr/bin\n', '11.0.0')
 ])
 def test_clang_version_detection(version_str, expected_version):
     version = spack.compilers.clang.Clang.extract_version_from_output(

var/spack/repos/builtin/packages/llvm/package.py

@@ -244,11 +244,11 @@ def filter_detected_exes(cls, prefix, exes_in_prefix):
     def determine_version(cls, exe):
         version_regex = re.compile(
             # Normal clang compiler versions are left as-is
-            r'clang version ([^ )]+)-svn[~.\w\d-]*|'
+            r'clang version ([^ )\n]+)-svn[~.\w\d-]*|'
             # Don't include hyphenated patch numbers in the version
             # (see https://github.com/spack/spack/pull/14365 for details)
-            r'clang version ([^ )]+?)-[~.\w\d-]*|'
+            r'clang version ([^ )\n]+?)-[~.\w\d-]*|'
-            r'clang version ([^ )]+)|'
+            r'clang version ([^ )\n]+)|'
             # LLDB
             r'lldb version ([^ )\n]+)|'
             # LLD