Instead of creating and destroying a SILProfiler
per TopLevelCodeDecl, setup a single profiler
for the top-level entry point function, and visit
all the TopLevelCodeDecls when mapping regions.
Previously, if all the coverage counters for a
given SIL function were optimized out, we would
have dropped the coverage map on the floor,
leading to inaccurate coverage reports in `-O`.
However LLVM allows us to emit the name data for
the coverage maps separately by defining an
`__llvm_coverage_names` global with the name data,
which is picked up by its instrumentation lowering
pass. Use this to continue to emit coverage maps
for functions that do not get emitted.
rdar://99813754
