Increase temp space in mtype.c/Type::getTypeInfoIdent(int)

Compiling druntime+phobos, this method is called 1245 times. 1104 times malloc()
is called - that's 89%! Increasing the size from 40 to 128, malloc() is only
called 128 times, which looks better. (Average length is 66.)
This may still too small - compiling some unit tests I have seen mangled names
with length up to 16745.

Alternatives are:
- use always malloc()
- use always alloca()

The same seems to be true variable out of type OutBuffer. But I do not have
collected the numbers here.
Issue 11503 - immutable return value from pure function implicitly converts to mutable.