Try simplifying LexicalScopes ownership again.

Committed initially in r207724-r207726 and reverted due to compiler-rt
crashes in r207732.

Instead, fix this harder with unordered_map and store the LexicalScopes
by value in the map. This did necessitate moving the definition of
LexicalScope above the definition of LexicalScopes.

Let's see how the buildbots/compilers tolerate unordered_map::emplace +
std::piecewise_construct + std::forward_as_tuple...