[PseudoTerminal] Fix PseudoTerminal MSVC release crash

[Ilod]

Summary

Since r278177, the Posix functions in PseudoTerminal::OpenFirstAvailableMaster on windows are now inlined LLVM_UNREACHABLE instead of return 0.
This causes __assume(0) to be inlined in OpenFirstAvailableMaster, allowing the optimizer to change code because this function should never be executed. In particular, on Visual 2015 Update 3 Win32 Release builds, the optimizer skips the if (error_str) test, causing a crash if error_str is nullptr.
The added #if !defined(LLDB_DISABLE_POSIX) restore the previous behaviour (which was doing nothing and returning true, as every function was returning 0, and prevent crashes.
The crash was 100% when launching a test x86 executable (built with clang, linked with lld-link) in lldb.
I don't know if there is another fix in progress (not calling the function on Win32?), but it seems to be called from several places, so it may be simpler to fix it in PseudoTerminal.

[Ilod]

Yeah, you're right, the previous code is used for reference was inconsistent here. Now I return false and set an error string. Also remove an hacky ifdef which was needed on previous implementation (because the master id was updated to 0), but now we leave it to -1, so it's not needed anymore.

[zturner]

LGTM as long as you've run the test suite and confirmed everything works.

lldb/trunk/source/Utility/PseudoTerminal.cpp

PseudoTerminal::~PseudoTerminal() {
   CloseSlaveFileDescriptor();
 }
 
 //----------------------------------------------------------------------
 // Close the master file descriptor if it is valid.
 //----------------------------------------------------------------------
 void PseudoTerminal::CloseMasterFileDescriptor() {
   if (m_master_fd >= 0) {
-// Don't call 'close' on m_master_fd for Windows as a dummy implementation of
-// posix_openpt above always gives it a 0 value.
-#ifndef _WIN32
     ::close(m_master_fd);
-#endif
     m_master_fd = invalid_fd;
   }
 }
 
 //----------------------------------------------------------------------
 // Close the slave file descriptor if it is valid.
 //----------------------------------------------------------------------
 void PseudoTerminal::CloseSlaveFileDescriptor() {

 // ownership of the master file descriptor can be released using
 // the ReleaseMasterFileDescriptor() accessor. If this object has
 // a valid master files descriptor when its destructor is called, it
 // will close the master file descriptor, therefore clients must
 // call ReleaseMasterFileDescriptor() if they wish to use the master
 // file descriptor after this object is out of scope or destroyed.
 //
 // RETURNS:
-//  Zero when successful, non-zero indicating an error occurred.
+//  True when successful, false indicating an error occurred.
 //----------------------------------------------------------------------
 bool PseudoTerminal::OpenFirstAvailableMaster(int oflag, char *error_str,
                                               size_t error_len) {
   if (error_str)
     error_str[0] = '\0';
 
+#if !defined(LLDB_DISABLE_POSIX)
   // Open the master side of a pseudo terminal
   m_master_fd = ::posix_openpt(oflag);
   if (m_master_fd < 0) {
     if (error_str)
       ::strerror_r(errno, error_str, error_len);
     return false;
   }
 
#if !defined(LLDB_DISABLE_POSIX)
   if (::unlockpt(m_master_fd) < 0) {
     if (error_str)
       ::strerror_r(errno, error_str, error_len);
     CloseMasterFileDescriptor();
     return false;
   }
 
   return true;
+#else
+  if (error_str)
+    ::snprintf(error_str, error_len, "%s",
+               "pseudo terminal not supported");
+  return false;
+#endif
 }
 
 //----------------------------------------------------------------------
 // Open the slave pseudo terminal for the current master pseudo
 // terminal. A master pseudo terminal should already be valid prior to
 // calling this function (see OpenFirstAvailableMaster()).
 // The file descriptor is stored this object's member variables and can
 // be accessed via the GetSlaveFileDescriptor(), or released using the
 // ReleaseSlaveFileDescriptor() member function.
 //
 // RETURNS:
-//  Zero when successful, non-zero indicating an error occurred.
+//  True when successful, false indicating an error occurred.
 //----------------------------------------------------------------------
 bool PseudoTerminal::OpenSlave(int oflag, char *error_str, size_t error_len) {
   if (error_str)
     error_str[0] = '\0';
 
   CloseSlaveFileDescriptor();
 
   // Open the master side of a pseudo terminal