Rudimentary name mangling support (#4267)

This seems to be enough to avoid naming collisions for functions in any
of the current test cases (verified by asserting that the name of the
`llvm::Function` matches the name passed to create it - not triggering
LLVM's numbering that happens when names collide)

It currently implements mangling for namespace scopes, class scopes, and
impls.
Nothing generic is mangled yet - haven't looked at how that works,
though evidently it's not covered by existing testing, I guess.

Follow-up change will document the current mangling algorithm in
`toolchain/docs/lower.md`

---------

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>
Co-authored-by: Carbon Infra Bot <carbon-external-infra@google.com>

toolchain/codegen/testdata/assembly/basic.carbon

@@ -5,6 +5,6 @@
 // ARGS: compile --no-prelude-import --target=x86_64-unknown-linux-gnu --output=- %s
 // NOAUTOUPDATE
 // SET-CHECK-SUBSET
-// CHECK:STDOUT: Main:
+// CHECK:STDOUT: _CMain.Main:
 
 fn Main() {}