[lldb][test] XFAIL TestClangModulesDeclLookup on Linux

Failing on the Linux bots with:
```
+ /home/worker/2.0.1/lldb-x86_64-debian/build/bin/FileCheck /home/worker/2.0.1/lldb-x86_64-debian/llvm-project/lldb/test/Shell/Expr/TestClangModulesDeclLookup.test
/home/worker/2.0.1/lldb-x86_64-debian/llvm-project/lldb/test/Shell/Expr/TestClangModulesDeclLookup.test:56:15: error: CHECK-NEXT: expected string not found in input
              ^
<stdin>:38:26: note: scanning from here
(lldb) expression foo(50)
                         ^
<stdin>:41:34: note: possible intended match here
(lldb) target modules dump ast --filter foo
                                 ^
```

Author: Michael137

lldb/test/Shell/Expr/TestClangModulesDeclLookup.test

@@ -1,4 +1,5 @@
 # XFAIL: target-windows
+# XFAIL: system-linux
 
 # Test that we can successfully locate decls in Clang modules for C++.