Add thinlto support to codegen, assembly and coverage tests
Using `--emit=llvm-ir` with thinlto usually result in multiple IR files.
Resolve test case failure issue reported in #113923.
In some cases we need to prepare a coverage expression that is the sum of an
arbitrary number of other terms. This patch simplifies the code paths that
build those sums.
This causes some churn in the mappings, because the previous code was building
its sums in a somewhat idiosyncratic order.
Without the workaround applied, this test will produce malformed mappings that
cause `llvm-cov` to fail.
(And if it does emit well-formed mappings, they should be obviously incorrect.)
