Move C-related declarations to compile-c.h

Move C-related declarations to compile-c.h

This patch simply moves a bunch of C language-related declarations from
the various compile header files into a new C-specific header, compile-c.h.

gdb/ChangeLog:
        * Makefile.in (SUBDIR_GCC_COMPILE_SRCS): Move header files ...
        (HFILES_NO_SRCDIR): ... to here.
        Add compile-internal.h and compile-c.h.
        * compile/compile-c-support.c: Include compile-c.h.
        * compile/compile-c-symbols.c: Include compile-c.h.
        (generate_c_for_variable_locations): Update comment.
        * compile/compile-c-types.c: Include compile-c.h.
        * compile/compile-c.h: New file -- moved C language declarations
        from other files here.
        * compile/compile-internal.h: Do not include hashtab.h or
        common/enum-flags.h.
        (gcc_qualifiers_flags, struct compile_c_instance, C_CTX)
        (gcc_convert_symbol, gcc_symbol_address)
        (generate_c_for_variable_locations, c_get_mode_for_size)
        (c_get_range_decl_name): Definitions moved to compile-c.h.
        * compile/compile-loc2c.c: Include compile-c.h.