Use read_memory in ada_exception_message_1

Use read_memory in ada_exception_message_1

Testing using the internal AdaCore test suite showed a regression from
the target string reading changes.  In particular, now
ada_exception_message_1 can get the wrong answer in some cases.  In
particular, when an Ada exception catchpoint is hit, sometimes the
exception name will be incorrect.  The case I was seeing changed from
the correct:

    Catchpoint 2, CONSTRAINT_ERROR (catch C_E) at [...]

to:

    Catchpoint 2, CONSTRAINT_ERROR (catch C_EE) at [...]

I was not able to reproduce this failure with the Fedora gnat.
Perhaps it is related to some local change to gnat; I do not know.

Meanwhile, because ada_exception_message_1 knows the length of the
string to read, we can use read_memory here.  This fixes the bug.

I've updated the test suite to at least exercise this code path.
However, as mentioned above, the new test does not actually provoke
the failure.

gdb/ChangeLog
2020-07-08  Tom Tromey  <tromey@adacore.com>

	* ada-lang.c (ada_exception_message_1): Use read_memory.

gdb/testsuite/ChangeLog
2020-07-08  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/catch_ex/foo.adb: Pass string to raise.
	* gdb.ada/catch_ex.exp: Examine catchpoint text.