[lldb][test] TestDataFormatterLibcxxInvalidVectorSimulator.py: fix inline namespace warnings

Fixes:
```
/Users/ec2-user/jenkins/workspace/apple-llvm-project-pr-macos/branch-swift/release/6.2/llvm-project/lldb/test/API/functionalities/data-formatter/data-formatter-stl/libcxx-simulators/invalid-vector/main.cpp:5:11: warning: inline namespace reopened as a non-inline namespace [-Winline-namespace-reopened-noninline]
    5 | namespace __1 {
      |           ^
```

Drive-by: compile test as C++20 (in an attempt to fix another buildbot issue)
(cherry picked from commit dfed3d2)

Author: Michael137

lldb/test/API/functionalities/data-formatter/data-formatter-stl/libcxx-simulators/invalid-vector/Makefile

@@ -1,3 +1,3 @@
 CXX_SOURCES := main.cpp
-override CXXFLAGS_EXTRAS += -std=c++14
+override CXXFLAGS_EXTRAS += -std=c++20
 include Makefile.rules

lldb/test/API/functionalities/data-formatter/data-formatter-stl/libcxx-simulators/invalid-vector/main.cpp

@@ -2,19 +2,19 @@
 #include <libcxx-simulators-common/compressed_pair.h>
 
 namespace std {
-namespace __1 {
+inline namespace __1 {
 template <typename T> struct vector {
   T *__begin_;
   T *__end_;
   _LLDB_COMPRESSED_PAIR(T *, __cap_ = nullptr, void *, __alloc_);
 };
 } // namespace __1
 
-namespace __2 {
+inline namespace __2 {
 template <typename T> struct vector {};
 } // namespace __2
 
-namespace __3 {
+inline namespace __3 {
 template <typename T> struct vector {
   T *__begin_;
   T *__end_;