[docs] Add DIA register instructions to Getting Started with Visual Studio page

[yaron.keren]

Summary

Since Visual Studio 2017 the DIA libs are not registered by default, see:
https://docs.microsoft.com/en-us/visualstudio/extensibility/breaking-changes-2017?view=vs-2019#change-reduce-registry-impact
LLDB building instruction already specify registering these DLLs, required
both the LLVM PDB tests and LLDB build.

[yaron.keren]

Address https://bugs.llvm.org/show_bug.cgi?id=51643

llvm/docs/GettingStartedVS.rst

 5. In the first install screen, select both **Install launcher for all users**
    and **Add Python to the PATH**. This will allow installing psutil for all
    users for the regression tests and make Python available from the command
    line.
 6. In the second install screen, select (again) **Install for all users** and
    if you want to develop `lldb <https://lldb.llvm.org/>`_, selecting
    **Download debug binaries** is useful.
 7. Complete the Python installation.
-8. Run a "Developer Command Prompt for VS 2019" as administrator. This command
+8. Run a "Developer Command Prompt for VS 2019" **as administrator**. This command
     prompt provides correct path and environment variables to Visual Studio and
     the installed tools.
 9. In the terminal window, type the commands:
 
    .. code-block:: bat
 
      c:
      cd \
 
   You may install the llvm sources in other location than ``c:\llvm`` but do not
   install into a path containing spaces (e.g. ``c:\Documents and Settings\...``)
   as it will fail.
 
-10. Install psutil and obtain LLVM source code:
+10. Register the Microsoft Debug Interface Access (DIA) DLLs
+
+    .. code-block:: bat
+
+     regsvr32 "%VSINSTALLDIR%\DIA SDK\bin\msdia140.dll"
+     regsvr32 "%VSINSTALLDIR%\DIA SDK\bin\amd64\msdia140.dll"
+
+ The DIA library is required for LLVM PDB tests and
+ `LLDB development <https://lldb.llvm.org/resources/build.html>`_.
+
+11. Install psutil and obtain LLVM source code:
 
     .. code-block:: bat
 
      pip install psutil
      git clone https://github.com/llvm/llvm-project.git llvm
  
  Instead of ``git clone`` you may download a compressed source distribution
  from the `releases page <https://github.com/llvm/llvm-project/releases>`_.
  Select the last link: ``Source code (zip)`` and unpack the downloaded file using
  Windows Explorer built-in zip support or any other unzip tool.
 
-11. Finally, configure LLVM using CMake:
+12. Finally, configure LLVM using CMake:
 
     .. code-block:: bat
 
        cmake -S llvm\llvm -B build -DLLVM_ENABLE_PROJECTS=clang -DLLVM_TARGETS_TO_BUILD=X86 -Thost=x64
        exit
 
    ``LLVM_ENABLE_PROJECTS`` specifies any additional LLVM projects you want to
    build while ``LLVM_TARGETS_TO_BUILD`` selects the compiler targets. If
12. Finally, configure LLVM using CMake:
    * CMake generates project files for all build types. To select a specific
      build type, use the Configuration manager from the VS IDE or the
      ``/property:Configuration`` command line option when using MSBuild.
    * By default, the Visual Studio project files generated by CMake use the
      32-bit toolset. If you are developing on a 64-bit version of Windows and
      want to use the 64-bit toolset, pass the ``-Thost=x64`` flag when
      generating the Visual Studio solution. This requires CMake 3.8.0 or later.
 
-12. Start Visual Studio and select configuration:
+13. Start Visual Studio and select configuration:
 
    In the directory you created the project files will have an ``llvm.sln``
    file, just double-click on that to open Visual Studio. The default Visual
    Studio configuration is **Debug** which is slow and generates a huge amount
    of debug information on disk. For now, we recommend selecting **Release**
    configuration for the LLVM project which will build the fastest or
    **RelWithDebInfo** which is also several time larger than Release.
    Another technique is to build all of LLVM in Release mode and change
    compiler flags, disabling optimization and enabling debug information, only
    for specific librares or source files you actually need to debug.
 
-13. Test LLVM in Visual Studio:
+14. Test LLVM in Visual Studio:
 
    You can run LLVM tests by merely building the project "check-all". The test
    results will be shown in the VS output window. Once the build succeeds, you
    have verified a working LLVM development environment!
    
    You should not see any unexpected failures, but will see many unsupported
    tests and expected failures: