Fix MSVC build with C++ EH enabled

Mark the CrashRecoveryContextImpl constructor noexcept, so that MSVC
won't emit an unwind helper to clean up the allocation from `new` if the
constructor throws an exception.

Otherwise, MSVC complains:
  llvm\lib\Support\CrashRecoveryContext.cpp(220): error C2712: \
  Cannot use __try in functions that require object unwinding

The other simple fix would be to wrap `new` in a static helper or
lambda.

Users have reported that Tensorflow builds LLVM with /EHsc.