Fix memory allocating inside signal handler (MI)

Summary:
This patch fixes a memory allocating inside signal handler.

This bug was found by @vharron:
> Hi all,
> 
> I noticed these thread sanitizer warnings while running lldb-mi tests on
> Linux.
> 
> WARNING: ThreadSanitizer: signal-unsafe call inside of a signal (pid=39721)
> 
>     #0 operator new(unsigned long) <null>:0 (lldb-mi-3.7.0+0x000000092b9d)
> 
>     #1 std::string::_Rep::_S_create(unsigned long, unsigned long,
> std::allocator<char> const&) <null>:0 (libstdc++.so.6+0x0000000ba3b8)
> 
>     #2 CMICmnResources::GetStringFromResource(unsigned int, CMIUtilString&)
> const
> /usr/local/google/home/vharron/ll/tot/llvm/tools/lldb/tools/lldb-mi/MICmnResources.cpp:434
> (lldb-mi-3.7.0+0x00000014ddd8)
> 
>     #3 CMICmnResources::GetString(unsigned int) const
> /usr/local/google/home/vharron/ll/tot/llvm/tools/lldb/tools/lldb-mi/MICmnResources.cpp:371
> (lldb-mi-3.7.0+0x00000014db81)
> 
>     #4 sigwinch_handler(int)
> /usr/local/google/home/vharron/ll/tot/llvm/tools/lldb/tools/lldb-mi/MIDriverMain.cpp:99
> (lldb-mi-3.7.0+0x0000001589ff)
> 
>     #5 __tsan::CallUserSignalHandler(__tsan::ThreadState*, bool, bool, int,
> my_siginfo_t*, void*) tsan_interceptors.o:0 (lldb-mi-3.7.0+0x00000009f25f)
> 
>     #6 void std::this_thread::sleep_for<long, std::ratio<1l, 1000l>
> >(std::chrono::duration<long, std::ratio<1l, 1000l> > const&)
> /usr/bin/../lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/thread:279
> (lldb-mi-3.7.0+0x00000013243e)
> 
>     #7 CMIDriver::ReadStdinLineQueue()
> /usr/local/google/home/vharron/ll/tot/llvm/tools/lldb/tools/lldb-mi/MIDriver.cpp:710
> (lldb-mi-3.7.0+0x000000155e62)
> 
>     #8 CMIDriver::DoMainLoop()
> /usr/local/google/home/vharron/ll/tot/llvm/tools/lldb/tools/lldb-mi/MIDriver.cpp:631
> (lldb-mi-3.7.0+0x000000155d37)
> 
>     #9 non-virtual thunk to CMIDriver::DoMainLoop()
> /usr/local/google/home/vharron/ll/tot/llvm/tools/lldb/tools/lldb-mi/MIDriver.cpp:648
> (lldb-mi-3.7.0+0x000000155fbd)
> 
>     #10 CMIDriverMgr::DriverMainLoop()
> /usr/local/google/home/vharron/ll/tot/llvm/tools/lldb/tools/lldb-mi/MIDriverMgr.cpp:340
> (lldb-mi-3.7.0+0x000000159ee6)
> 
>     #11 main
> /usr/local/google/home/vharron/ll/tot/llvm/tools/lldb/tools/lldb-mi/MIDriverMain.cpp:364
> (lldb-mi-3.7.0+0x000000158f60)

Reviewers: vharron, abidh

Reviewed By: abidh

Subscribers: lldb-commits, abidh, vharron

Differential Revision: http://reviews.llvm.org/D8256

llvm-svn: 232735