[lldb] Don't use __FUNCTION__ as a file name

[teemperor]

Summary

I saw while debugging that we call this file ParseInternal, which is not a very good name for our
fake expression file and also adds this unnecessary link between the way we name this function
and the other source location names we get from the expression parser. This patch is renaming
it to <lldb-expr> which is closer to the way Clang names its buffers, it doesn't depend on the
function name (which changes when I refactor this code) and it's easier to grep for.

[davide]

This seems fairly straighforward and I'm in favor of it. We maybe should do the same thing with swift, I haven't checked in a while how we call the file there.
I wouldn't bother adding a test for something so minor.

lldb/source/Plugins/ExpressionParser/Clang/ClangExpressionParser.cpp

if (temp_fd != -1) {
           created_main_file = true;
         }
       }
     }
   }
 
   if (!created_main_file) {
     std::unique_ptr<MemoryBuffer> memory_buffer =
-        MemoryBuffer::getMemBufferCopy(expr_text, __FUNCTION__);
+        MemoryBuffer::getMemBufferCopy(expr_text, "<lldb-expr>");
     source_mgr.setMainFileID(source_mgr.createFileID(std::move(memory_buffer)));
   }
 
   diag_buf->BeginSourceFile(m_compiler->getLangOpts(),
                             &m_compiler->getPreprocessor());
 
   ClangExpressionHelper *type_system_helper =
       dyn_cast<ClangExpressionHelper>(m_expr.GetTypeSystemHelper());