Allow all valid C declaration syntax inside the EXEC SQL declare section

When the preprocessor runs on a file, it expands all the contents of header files inside the source file where the header file was included in the source file. Therefore, one consequence of including C header files inside the EXEC SQL declare section is that all types of C declaration syntax are included in the EXEC SQL declare section after they pass through the C preprocessor. You can now include all valid C declaration syntax in the EXEC SQL declare section in the EXEC SQL declare section. However, you can only declare host variables based on certain types described in Host-variable data types.