Stop wrapping GCCAsmStmts inside StmtExprs to destruct temporaries

Instead, just pop the cleanups at the end of the asm statement.

This fixes an assertion failure in BuildStmtExpr. It also fixes a bug
where blocks and C compound literals were destructed at the end of the
asm statement instead of at the end of the enclosing scope.

Differential Revision: https://reviews.llvm.org/D125936