Sema: Fix logic error in TypeChecker::typeCheckPattern()

Use overwriteType() instead of setType() because we can end up with
a type here if the pattern was previously marked invalid.

I don't have a test for this, but its needed to fix some
compiler_crashers regressions with upcoming patches.

Author: slavapestov

lib/Sema/TypeCheckPattern.cpp

@@ -784,7 +784,8 @@ bool TypeChecker::typeCheckPattern(Pattern *P, DeclContext *dc,
     P->setType(ErrorType::get(Context));
     if (auto named = dyn_cast<NamedPattern>(P)) {
       if (auto var = named->getDecl()) {
-        var->setType(ErrorType::get(Context));
+        var->setInvalid();
+        var->overwriteType(ErrorType::get(Context));
       }
     }
     return true;