[windows] Enable tests for LLDB in Swift CI (swiftlang#78844)

TL;DR Please revert this patch in case LLDB testing on Windows clogs Swift PR-tests

The LLDB test suite is not very stable on Windows, which is a problem when we want to run them in CI. In order to improve the situation, however, we need to get it tested continuously. This patch attempts to solve the chicken-egg problem. It introduces a simple override for unstable tests: list them in file `utils/windows-llvm-lit-test-overrides.txt` and SwiftCI will skip/xfail them.

This approach has a number of benefits:
* overrides don't need to be in sync with https://github.com/swiftlang/llvm-project
* overrides are tracked in one place and they are not spread across the LLDB test suite
* overrides are swiftlang-specific, which clearly states the differences to upstream LLVM
* we can enable continuous testing (and get reports for new failures) without fixing the world first

Once the remaining subset of tests passes reliably, we can iterate to enable the others and increase coverage. My previous patch 459e592 implemented the necessary infrastructure. This one only enable tests in CI. The separation allows to disable LLDB testing on Windows with fewer side-effects.

Author: weliveindetail

utils/build-windows-toolchain.bat

@@ -60,7 +60,7 @@ set TMPDIR=%BuildRoot%\tmp
 set NINJA_STATUS=[%%f/%%t][%%p][%%es] 
 
 :: Build the -Test argument, if any, by subtracting skipped tests
-set TestArg=-Test lld,swift,dispatch,foundation,xctest,swift-format,sourcekit-lsp,
+set TestArg=-Test lld,lldb,swift,dispatch,foundation,xctest,swift-format,sourcekit-lsp,
 for %%I in (%SKIP_TESTS%) do (call set TestArg=%%TestArg:%%I,=%%)
 if "%TestArg:~-1%"=="," (set TestArg=%TestArg:~0,-1%) else (set TestArg= )