Prevent infinite recursive loop in AppleObjCTrampolineHandler constructor

Summary:
When we construct AppleObjCTrampolineHandler, if m_impl_fn_addr is
invalid, we call CanJIT(). If the gdb remote process does not support
allocating and deallocating memory, this call stack will include a call
to the AppleObjCRuntime constructor. The AppleObjCRuntime constructor
will then call the AppleObjCTrampolineHandler constructor, creating a
recursive call loop that eventually overflows the stack and segfaults.

Avoid this call loop by not constructing the AppleObjCTrampolineHandler
within AppleObjCRuntime until we actually need to use it.

Reviewers: clayborg, jingham

Subscribers: sas, lldb-commits

Differential Revision: http://reviews.llvm.org/D15978

Change by Francis Ricci <fjricci@fb.com>

llvm-svn: 257204