Although users should usually use a prebuilt standard library, in those rare configurations where one needs to be built, the compiler appears to hang for several minutes, even on a trivial compilation. This commit adds a remark that's emitted when this happens, explaining that the standard library is being rebuilt and it will take a few minutes.
Previously, when the standard library module interface was broken, Swift would try to rebuild it repeatedly during -compile-module-from-interface jobs because `ASTContext::getStdlibModule()` would try to load the standard library again each time it was called. This led to extremely slow compilations that repeatedly emitted the same errors.
To avoid this, we make ModuleInterfaceBuilder try to load the standard library right away and bail out if it can’t.
Fixes rdar://75669548.
