Use enum bitfield for the calling_convention attribute of a subroutine

This is a refactoring.  Instead of a plain unsigned value, use an enum
bitfield.

gdb/ChangeLog:
2019-12-20  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* dwarf2read.c (is_valid_DW_AT_calling_convention_for_subroutine):
	New function.
	(read_subroutine_type): Validate the parsed
	DW_AT_calling_convention value before assigning it to a
	subroutine's calling_convention attribute.
	* gdbtypes.h (struct func_type) <calling_convention>: Use
	an enum bitfield as its type, instead of plain unsigned.

Change-Id: Ibc6b2f71e885cbc5c3c9d49734f7125acbfd1bcd