[flang][NFC] Restore I/O runtime API header name

flang/include/flang/Runtime/io-api.h was changed into io-api-consts.h,
then wrapped into a new io-api.h that includes io-api-consts.h, does
some redundant includes and declarations, and then declares the prototype
of one function, InquiryKeywordHashDecode.

Make that function static in io-stmt.cpp prior to its sole call site,
then undo the renaming, to reduce confusion and redundancy.

Author: klausler

flang/include/flang/Optimizer/Builder/Runtime/RTBuilder.h

@@ -21,7 +21,7 @@
 #include "flang/Optimizer/Builder/FIRBuilder.h"
 #include "flang/Optimizer/Dialect/FIRDialect.h"
 #include "flang/Optimizer/Dialect/FIRType.h"
-#include "flang/Runtime/io-api-consts.h"
+#include "flang/Runtime/io-api.h"
 #include "flang/Runtime/reduce.h"
 #include "flang/Support/Fortran.h"
 #include "mlir/IR/BuiltinTypes.h"