gdb: Convert language_data::string_lower_bound to a method

gdb: Convert language_data::string_lower_bound to a method

Convert language_data::string_lower_bound member variable to a virtual
method language_defn::string_lower_bound.

Over all of the languages we currently support there are currently
only two values for the lower bound, 0 or 1.  I noticed that in all
cases, if a language has C style arrays then the lower bound is 0,
otherwise the lower bound is 1.  So the default for the virtual method
in language.h makes use of this, which means languages don't have to
worry about providing a string_lower_bound method at all.

Except for Modula2.  This language is defined to not have C style
arrays, but has a string_lower_bound index of 0, this behaviour is
maintained after this commit by having Modula2 be the only language
that overrides the string_lower_bound method.

There should be no user visible changes after this commit.

gdb/ChangeLog:

	* ada-lang.c (ada_language_data): Remove string_lower_bound
	initializer.
	* c-lang.c (c_language_data): Likewise.
	(cplus_language_data): Likewise.
	(asm_language_data): Likewise.
	(minimal_language_data): Likewise.
	* d-lang.c (d_language_data): Likewise.
	* f-lang.c (f_language_data): Likewise.
	* go-lang.c (go_language_data): Likewise.
	* language.c (unknown_language_data): Likewise.
	(auto_language_data): Likewise.
	* language.h (language_data): Remove string_lower_bound field.
	(language_defn::string_lower_bound): New member function.
	* m2-lang.c (m2_language_data): Remove string_lower_bound
	initializer.
	(m2_language::string_lower_bound): New member function.
	* objc-lang.c (objc_language_data): Remove string_lower_bound
	initializer.
	* opencl-lang.c (opencl_language_data): Likewise.
	* p-lang.c (pascal_language_data): Likewise.
	* rust-lang.c (rust_language_data): Likewise.
	* valops.c (value_cstring): Update call to string_lower_bound.
	(value_string): Likewise.
	* value.c (allocate_repeated_value): Likewise.