LLVM is inconsistent about how it converts `errno` to `std::error_code`.
This can cause problems because values outside of `std::errc` compare
differently if one is system and one is generic on POSIX systems.

This is even more of a problem on Windows where use of the system
category is just wrong, as that is for Windows errors, which have a
completely different mapping than POSIX/generic errors. This patch fixes
one instance of this mistake in `JSONTransport.cpp`.

This patch adds `errnoAsErrorCode()` which makes it so people do not
need to think about this issue in the future. It also cleans up a lot of
usage of `errno` in LLVM and Clang.

Relands #81708, which was reverted by
f410f74cd5b26319b5796e0404c6a0f3b5cc00a5, now with a corrected unit
test. Origionally the test failed on Windows when run with lit as
`GetConsoleWindow` could not retrieve a window handle regardless of
whether `DetachProcess` was `true` or `false`. The test now uses
`GetStdHandle(STD_OUTPUT_HANDLE)` which does not rely on a console
window existing. Original commit message below.

Adds a new parameter, `bool DetachProcess` with a default option of
`false`, to `llvm::sys::ExecuteNoWait`, which, when set to `true`,
executes the specified program without a controlling terminal.

Functionality added so that the module build daemon can be run without a
controlling terminal.

This reverts commit 86f6caa562255f81b93e72a501a926b17f5ad244. Unit test
was failing on a few windows build bots

Adds a new parameter, `bool DetachProcess` with a default option of
`false`, to `llvm::sys::ExecuteNoWait`, which, when set to `true`,
executes the specified program without a controlling terminal.

Functionality added so that the module build daemon can be run without a
controlling terminal.

Currently the process is terminated after the timeout. Add an option
to let the process resume after the timeout instead.

https://reviews.llvm.org/D138952

I found the interaction between SecondsToWait and
WaitUntilChildTerminates confusing. Rather than have a boolean to
ignore the value of SecondsToWait, combine these into one Optional
parameter.

Apply clang-format on llvm/lib/Support/Windows/ and llvm/lib/Support/Unix/ since .inc files in these folders aren't picked up by default. Eventually we need to add this extension in the monorepo .clang-format file.

Differential Revision: https://reviews.llvm.org/D138714

This also includes a few cleanup from Support.

Discourse thread: https://discourse.llvm.org/t/include-what-you-use-include-cleanup
Differential Revision: https://reviews.llvm.org/D121331

Replace a few `reserve()` / `set_size()` pairs with
`resize_for_overwrite()` / `truncate()` in the platform-specific
code for Windows.

Differential Revision: https://reviews.llvm.org/D115390