Refactor single-unit checking out of check.cpp (#4649)

This is primarily moving code around, to try to create a logical split
of the code in check.cpp, makingthe API boundaries clearer.

There's one small, deliberate logic change around false returns from
`HandleParseNode`, where before there was a `CARBON_CHECK` instantiated
by the `#define` (per `NodeKind`), and now it's outside the `#define`
(done mainly because the message didn't keep up with the `Handle##Name`
-> `HandleParseNode` rename).

toolchain/check/BUILD

@@ -96,6 +96,8 @@ cc_library(
     name = "check",
     srcs = [
         "check.cpp",
+        "check_unit.cpp",
+        "check_unit.h",
         "deferred_definition_worklist.cpp",
         "deferred_definition_worklist.h",
         "handle.h",