dependencies/llvm: Mark as not found when not found

When either the shard or static libs are not available, and the
dependency is not required mark the dependency as not found and return.

Fixes #4360

mesonbuild/dependencies/dev.py

@@ -310,6 +310,7 @@ class LLVMDependency(ConfigToolDependency):
                 if not matches:
                     if self.required:
                         raise
+                    self.is_found = False
                     return
 
                 self.link_args = self.get_config_value(['--ldflags'], 'link_args')
@@ -326,6 +327,8 @@ class LLVMDependency(ConfigToolDependency):
             except DependencyException:
                 if self.required:
                     raise
+                self.is_found = False
+                return
 
         link_args = ['--link-static', '--system-libs'] if self.static else ['--link-shared']
         self.link_args = self.get_config_value(