[PATCH] Update testsuite mechanism to allow object files as source files.

[Caroline Tice]

Since this patch will be resulting in something of a policy change, I
expect there will be a fair
amount of discussion around it.

High level summary:
In some (hopefully rare) cases, certain tests MUST be built with a
particular compiler to work.  If that compiler is not GCC, then it
makes it very burdensome for developers to run the GDB testsuite and
test those tests.  This change allows those tests to use object files
(generated by the required compiler) as the test source file, so that
when the test suite is run with GCC those tests will still
execute/test properly.

Details:
This change is motivated by the fact that clang and GCC sometimes generate
different output, which affects whether or not a test works properly.  The
particular motivating example is the gdb.dwarf2/dw5-rnglist-test.exp test,
which tests whether or not GDB is properly reading/handling DWARF v5
DW_AT_ranges of the form DW_FORM_rnglistx in lexical block dies.  GCC does
not generate that code, so compiling the test with GCC does not successfully
test anything. Clang does generate that code, so the only way to properly
test this is to compile the test case with clang.  A further twist is that
the GNU assembler currently chokes on DWARF v5 .debug_line sections, because
they start their file index at 0 instead of 1, and the GNU assembler has not
been updated to handle this.  This is not a problem for
assembly files generated by GCC, because GCC generates DWARF v3
.debug_line sections, even when passed -gdwarf-5.  Clang on the other hand,
when passed -gdwarf-5, generates DWARF v5 .debug_line tables. So using a
clang-generated .s file for the test is not an option, because the GNU
assembler chokes on it.

This patch updates lib/gdb.exp to allow it to properly handle test
cases where the source file is in fact an object file, and it updates
the dw5-rnglist-test to use an object file rather than the .cc file.  This
in turn makes the test itself compiler-agnostic.


-- Caroline Tice
cmtice@google.com

gdb/testsuite/ChangeLog

2020-07-16  Caroline Tice  <cmtice@google.com>

        * lib/gdb.exp (build_executable_from_specs): Create output_dir
        variable from call to standard_output_file; update code that
        compiles source files to object files and copies them to output
        directory, to check first and make sure they are not already
        object file.  If they are already object files, just copy them to
        output directory.
        * gdb.dwarf2/dw5-rnglist-test.o: New file (object file)
        * gdb.dwarf2/dw5-rnglist-test.exp: Update to use .o file rather than
        .cc file; this removes the dependency of this test on clang and
        allows it to work with GCC as well.
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