[Reland][Debuginfo][llvm-dwarfutil] llvm-dwarfutil dsymutil-like tool for ELF.

This patch implements proposal https://lists.llvm.org/pipermail/llvm-dev/2020-August/144579.html
llvm-dwarfutil - is a tool that is used for processing debug info(DWARF) located in built binary files to improve debug info quality, reduce debug info size. The patch currently implements smaller set of command-line options(comparing to the proposal):

```
./llvm-dwarfutil [options] <input file> <output file>

  --garbage-collection    Do garbage collection for debug info(default)
  -j <value>              Alias for --num-threads
  --no-garbage-collection Don`t do garbage collection for debug info
  --no-odr-deduplication  Don`t do ODR deduplication for debug types
  --no-odr                Alias for --no-odr-deduplication
  --no-separate-debug-file
                          Create single output file, containing debug tables(default)
  --num-threads <threads> Number of available threads for multi-threaded execution. Defaults to the number of cores on the current machine
  --odr-deduplication     Do ODR deduplication for debug types(default)
  --odr                   Alias for --odr-deduplication
  --separate-debug-file   Create two output files: file w/o debug tables and file with debug tables
  --tombstone [bfd,maxpc,exec,universal]
                          Tombstone value used as a marker of invalid address(default: universal)
    =bfd - Zero for all addresses and [1,1] for DWARF v4 (or less) address ranges and exec
    =maxpc - Minus 1 for all addresses and minus 2 for DWARF v4 (or less) address ranges
    =exec - Match with address ranges of executable sections
    =universal - Both: bfd and maxpc
```

Reviewed By: clayborg

Differential Revision: https://reviews.llvm.org/D86539

1 files changed, 122 insertions, 0 deletions

diff --git a/llvm/docs/CommandGuide/llvm-dwarfutil.rst b/llvm/docs/CommandGuide/llvm-dwarfutil.rst
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+llvm-dwarfutil - A tool to copy and manipulate debug info
+=========================================================
+
+.. program:: llvm-dwarfutil
+
+SYNOPSIS
+--------
+
+:program:`llvm-dwarfutil` [*options*] *input* *output*
+
+DESCRIPTION
+-----------
+
+:program:`llvm-dwarfutil` is a tool to copy and manipulate debug info.
+
+In basic usage, it makes a semantic copy of the input to the output. If any
+options are specified, the output may be modified along the way, e.g.
+by removing unused debug info.
+
+If "-" is specified for the input file, the input is read from the program's
+standard input stream. If "-" is specified for the output file, the output
+is written to the standard output stream of the program.
+
+The tool is still in active development.
+
+COMMAND-LINE OPTIONS
+--------------------
+
+.. option:: --garbage-collection
+
+ Removes pieces of debug information related to discarded sections.
+ When the linker does section garbage collection the abandoned debug info
+ is left behind. Such abandoned debug info references address ranges using
+ tombstone values. Thus, when this option is specified, the tool removes
+ debug info which is marked with the tombstone value.
+
+ That option is enabled by default.
+
+.. option:: --odr-deduplication
+
+ Remove duplicated types (if "One Definition Rule" is supported by source
+ language). Keeps first type definition and removes other definitions,
+ potentially significantly reducing the size of output debug info.
+
+ That option is enabled by default.
+
+.. option:: --help, -h
+
+ Print a summary of command line options.
+
+.. option:: --no-garbage-collection
+
+ Disable :option:`--garbage-collection`.
+
+.. option:: --no-odr-deduplication
+
+ Disable :option:`--odr-deduplication`.
+
+.. option:: --no-separate-debug-file
+
+ Disable :option:`--separate-debug-file`.
+
+.. option:: --num-threads=<n>, -j
+
+ Specifies the maximum number (`n`) of simultaneous threads to use
+ for processing.
+
+.. option:: --separate-debug-file
+
+ Generate separate file containing output debug info. Using
+ :program:`llvm-dwarfutil` with that option equals to the
+ following set of commands:
+
+.. code-block:: console
+
+ :program:`llvm-objcopy` --only-keep-debug in-file out-file.debug
+ :program:`llvm-objcopy` --strip-debug in-file out-file
+ :program:`llvm-objcopy` --add-gnu-debuglink=out-file.debug out-file
+
+.. option:: --tombstone=<value>
+
+ <value> can be one of the following values:
+
+   - `bfd`: zero for all addresses and [1,1] for DWARF v4 (or less) address ranges and exec.
+
+   - `maxpc`: -1 for all addresses and -2 for DWARF v4 (or less) address ranges.
+
+   - `universal`: both `bfd` and `maxpc`.
+
+   - `exec`: match with address ranges of executable sections.
+
+   The value `universal` is used by default.
+
+.. option:: --verbose
+
+ Enable verbose logging. This option disables multi-thread mode.
+
+.. option:: --verify
+
+ Run the DWARF verifier on the output DWARF debug info.
+
+.. option:: --version
+
+ Print the version of this program.
+
+SUPPORTED FORMATS
+-----------------
+
+The following formats are currently supported by :program:`llvm-dwarfutil`:
+
+ELF
+
+EXIT STATUS
+-----------
+
+:program:`llvm-dwarfutil` exits with a non-zero exit code if there is an error.
+Otherwise, it exits with code 0.
+
+BUGS
+----
+
+To report bugs, please visit <https://github.com/llvm/llvm-project/labels/tools:llvm-dwarfutil/>.