`debug::LocalVariable` expects the parent scope to contain a `debug::LexicalScope`. If it doesn't, then the variable is never registered, and doesn't show up in the watch windows.

DXC emits `DebugLocalVariable`s for parameters, with the parent scope set to the immediate function, with no lexical block.

To make parameters show up, make `debug::Function` inherit from `debug::LexicalBlock`. This makes all the existing logic that uses `debug::find<debug::LexicalBlock>()` work for both `DebugLexicalBlock` and `DebugFunction` scope types.

Fixes: b/170320768
Change-Id: I296353eae9bb7f04bc649174b9f33a29d1dcc6e7
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/49088
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Ben Clayton <bclayton@google.com>