Add basic CDB support to debuginfo compiletest s, to help catch `*.natvis` regression...

Add basic CDB support to debuginfo compiletest s, to help catch `*.natvis` regressions, like those fixed in #60687.

Several Microsoft debuggers (VS, VS Code, WinDbg, CDB, ...) consume the `*.natvis` files we embed into rust `*.pdb` files.
While this only tests CDB, that test coverage should help for all of them.

CHANGES

src\bootstrap
  - test.rs:  Run CDB debuginfo tests on MSVC targets

src\test\debuginfo
  - issue-13213.rs:  CDB has trouble with this, skip for now (newly discovered regression?)
  - pretty-std.rs:  Was ignored, re-enable for CDB only to start with, add CDB tests.
  - should-fail.rs:  Add CDB tests.

src\tools\compiletest:
  - Added "-cdb" option
  - Added Mode::DebugInfoCdb ("debuginfo-cdb")
  - Added run_debuginfo_cdb_test[_no_opt]
  - Renamed Mode::DebugInfoBoth -> DebugInfoGdbLldb ("debuginfo-gdb+lldb") since it's no longer clear what "Both" means.
  - Find CDB at the default Win10 SDK install path "C:\Program Files (x86)\Windows Kits\10\Debugger\*\cdb.exe"
  - Ignore CDB tests if CDB not found.

ISSUES

  - `compute_stamp_hash`: not sure if there's any point in hashing `%ProgramFiles(x86)%`
  - `OsString` lacks any `*.natvis` entries (would be nice to add in a followup changelist)
  - DSTs (array/string slices) which work in VS & VS Code fail in CDB.
  - I've avoided `Mode::DebugInfoAll` as 3 debuggers leads to pow(2,3)=8 possible combinations.

REFERENCE

CDB is not part of the base Visual Studio install, but can be added via the Windows 10 SDK:
  https://developer.microsoft.com/en-us/windows/downloads/windows-10-sdk
Installing just "Debugging Tools for Windows" is sufficient.

CDB appears to already be installed on appveyor CI, where this changelist can find it, based on it's use here:
  https://github.com/rust-lang/rust/blob/0ffc57311030a1930edfa721fe57d0000a063af4/appveyor.yml#L227

CDB commands and command line reference:
  https://docs.microsoft.com/en-us/windows-hardware/drivers/debugger/debugger-reference