[lldb-tests] Remove dubious standard library flag

[fdeazeve]

Summary

The test currently sets USE_LIBSTDCPP = 0, which is curious given the
behavior of and and or in Makefiles (the contents of the variables
are not important). In particular, this causes the tests to not use the
standard libraries appropriately.

To capture the actual intent of the test, we're changing this to
USE_LIBCXX=1.

[aprantl]

This looks obviously correct.

[fdeazeve]

This linux bot failed: https://lab.llvm.org/buildbot/#/builders/68/builds/41314
I've reverted while I give it a try in a linux VM. Interestingly, the standard library flags are not being passed correctly according to the log.

lldb/test/API/functionalities/data-formatter/data-formatter-skip-summary/Makefile

 CXX_SOURCES := main.cpp
-USE_LIBSTDCPP := 0
-
-
+USE_LIBCPP := 1
 
 CXXFLAGS_EXTRAS := -O0
 include Makefile.rules