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

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

@@ -243,11 +243,11 @@ 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 ([^ )]+)|'
+            r'clang version ([^ )\n]+?)-[~.\w\d-]*|'
+            r'clang version ([^ )\n]+)|'
             # LLDB
             r'lldb version ([^ )\n]+)|'
             # LLD