Fix dwarf2_name caching bug

Fix dwarf2_name caching bug

PR gdb/25663 points out that dwarf2_name will cache a value in the
bcache and then return a substring.  However, this substring return is
only done on the branch that caches the value -- so if the function is
called twice with the same arguments, it will return different values.

This patch fixes this problem.

This area is strange.  We cache the entire demangled string, but only
return the suffix.  I looked at caching just the suffix, but it turns
out that anonymous_struct_prefix assumes that the entire string is
stored.  Also weird is that this code is demangling the linkage name
and then storing the demangled form back into the linkage name
attribute -- that seems bad, because what if some code wants to find
the actual linkage name?

Fixing these issues was non-trivial, though; and in the meantime this
patch seems like an improvement.  Regression tested on x86-64
Fedora 30.

gdb/ChangeLog
2020-03-16  Tom Tromey  <tromey@adacore.com>

	PR gdb/25663:
	* dwarf2/read.c (dwarf2_name): Strip leading namespaces after
	putting value into bcache.