Don't re-process a DIE in read_lexical_block_scope

Don't re-process a DIE in read_lexical_block_scope

A customer reported a crash in the DWARF reader.

Investigation showed that the crash occurred in an unusual scenario: a
function was lexically scoped within some other function -- but the
inner function inlined the outer function and referred to its DIE via
DW_AT_abstract_origin.  With the executable in question,
inherit_abstract_dies could eventually call read_lexical_block_scope,
which in turn could recurse into process_die, to process a DIE that
was already being read, triggering an assert.

This came up once before; see:

https://www.sourceware.org/ml/gdb-patches/2014-02/msg00652.html

However, in this case, I don't have an easy way to reproduce.  So,
there is no test case.

I did experiment with the failing executable.  This patch fixes the
bug and doesn't seem to cause other issues.  For example, I can still
set breakpoints on the relevant functions.

gdb/ChangeLog
2020-05-08  Tom Tromey  <tromey@adacore.com>

	* dwarf2/read.c (read_lexical_block_scope): Don't process a DIE
	already being processed.