Write LF_BCLASS records in CodeView struct definitions

Write LF_BCLASS records in CodeView struct definitions

When writing the CodeView type definition for a struct, translate
DW_TAG_inheritance DIEs into LF_BCLASS records, to record which other
structs this one inherits from.

gcc/
	* dwarf2codeview.cc (enum cv_leaf_type): Add LF_BCLASS.
	(struct codeview_subtype): Add lf_bclass to union.
	(write_cv_padding): Add declaration.
	(write_lf_fieldlist): Handle LF_BCLASS records.
	(add_struct_inheritance): New function.
	(get_type_num_struct): Call add_struct_inheritance.