[lldb] Avoid calling raw_string_ostream::str() in LLDBAssert.cpp (NFC)

Revert to the state after d779685 and use the underlying buffer
directly. I was still under the impression that was unsafe, so I did a
drive-by fix, which this commit reverts.

(cherry picked from commit 96ef428)

Author: JDevlieghere

lldb/source/Utility/LLDBAssert.cpp

@@ -53,7 +53,7 @@ void _lldb_assert(bool expression, const char *expr_text, const char *func,
       llvm::formatv("Assertion failed: ({0}), function {1}, file {2}, line {3}",
                     expr_text, func, file, line)
           .str(),
-      backtrace.str(),
+      buffer,
       "Please file a bug report against lldb reporting this failure log, and "
       "as many details as possible");
 }