The previous lazy discovery did not always work because sometimes a debug_value
is emitted before the first SIL instruction in the variable's scope.
rdar://110841130
ConditionalClauseInitializerScope often create redundant scopes, however, they
are needed to correctly represent the scopes in an expression such as:
if case .something(let foo), let foo = foo.
This patch changes SILGen to lower them 1:1 from ASTScopes.
rdar://110841130
The previous code made the assumption that the ASTScope for a variable
declaration should be the one of the declaration's source location. That is not
necessarily the case, in some cases it should be an ancestor scope. This patch
introduces a map from ValueDecl -> ASTScope that is derived from querying each
ASTScope for its locals, which matches also what happens in name lookup. This
patch also fixes the nesting of SILDebugScopes created for guard statement
bodies, which are incorrectly nested in the ASTScope hierarchy.
rdar://108940570
