libiberty: Demangling 'M' prefixes

libiberty: Demangling 'M' prefixes

The grammar for a lambda context can include <source-name> 'M', and we
were adding the component that generated to the substitution table
twice.  Just ignore the 'M' completely -- we'll already have done the
checks we need when we saw its predecessor.  A prefix cannot be the
last component of a nested name, so we do not need to check for that
case (although we could if we wanted to be more lenient).

	libiberty/
	* cp-demangle.c (d_prefix): 'M' components are not
	(re-)added to the substitution table.
	* testsuite/demangle-expected: Add tests.