Clang: Disable printing stack traces by default

Printing the stack strace in the event of a crash is very useful, but in
the worst case it can also result in a deadlock, without any trace being
printed, rendering the clangbackend completely useless.

For example, if a thread allocates memory (lock) and another thread
crashes right then, clang's stack trace printer is invoked, which
(unfortunately) also tries to allocate memory (deadlock). This was
observed with QTCREATORBUG-21559.

The stack trace printer should be fixed in clang. Until then, disable
printing stack traces by default and allow to enable when explicitly
requested (QTC_CLANG_ENABLE_STACKTRACES=1).

Task-number: QTCREATORBUG-21559
Change-Id: I418cea7f3d70bc8a9cae333b3858228650307c6e
Reviewed-by: Christian Stenger <christian.stenger@qt.io>
Reviewed-by: Marco Bubke <marco.bubke@qt.io>
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

src/tools/clangbackend/clangbackendmain.cpp

@@ -80,6 +80,8 @@ int main(int argc, char *argv[])
 
     const QString connection = processArguments(application);
 
+    // Printing the stack strace might dead lock as clang's stack printer allocates memory.
+    if (qEnvironmentVariableIntValue("QTC_CLANG_ENABLE_STACKTRACES"))
         clang_enableStackTraces();
 
     ClangCodeModelServer clangCodeModelServer;