When importing Carbon code that refers to a C++ class, look for a
corresponding C++ class in the current context and import that instead.
This is a workaround for not having proper cross-file C++ import
support. For now, we only support non-templated namespace-scope class
types.
Assisted-by: Gemini via Antigravity
Instead of treating all C++ code as coming from a single synthetic
`CheckIRId`, track the `SemIR::File` associated with each C++ location.
This is necessary since each `SemIR::File` has a distinct `CppFile` and
therefore distinct `SourceLocation`s and `ClangSourceLocId`s.
Assisted-by: Gemini via Antigravity
Add import/ and export/ under function/. Move most top-level tests to a
new basics/ with subdirectories for `import` directives and `inline
Cpp`. Add subdirectory for primitive type handling. Move all `reverse/`
tests to somewhere else, typically under an `export/` directory.
I split two test files up: constexpr.carbon got split into var/ and
function/ pieces, and reverse/simple.carbon was inlined into
namespace/export.carbon. The rest are just simple renames.
Richard Smith