The ClangModulesDeclVendor is currently interpreting all injected `@import` statements in our expression
wrapper as modules that the user has explicitly requested to be persistently loaded. As we inject
`@import` statements with our std module prototype, the ClangModulesDeclVendor will start compiling
and loading unrelated C++ modules because it thinks the user has requested that it should load them. As
the ClangModulesDeclVendor is lacking the setup to compile these modules (e.g. it lacks the include paths),
it will then actually just fail to compile them and cause the whole expression evaluation to fail. This causes
these tests to fail on systems that enable the ClangModulesDeclVendor (such as macOS).
This patch fixes this by preventing the ClangModulesDeclVendor from interpreting `@import` statements
in the wrapper source code. This is done by defining a macro directly before the actual user expression and
letting the preprocessor callbacks ignore import statements until we hit that macro.
This patch also enables most of the `import-std-module/` tests to make sure this patch is working as
intended. I didn't enable the sysroot test here as it still fails (due to another issue which will be a separate patch).