gdb: Change psymbol_functions::require_partial_symbols to partial_symbols

gdb: Change psymbol_functions::require_partial_symbols to partial_symbols

The previous patch ensured that partial symbols are read before calling
most of the quick_function's methods.

The psymbol_functions class has the require_partial_symbols method which
serves this exact purpose, and does not need to do it anymore.

This patch renames this method to partial_symbols and makes it an accessor
which asserts that partial symbols have been read at this point.

Regression tested on x86_64-linux.