Since new headergen is now the default for building LLVM-libc, the docs
need to be updated to reflect that. While I was editing those docs, I
took a quick pass at updating other out-of-date pages.
This PR changes several places in the CMake scripts to make libc build
on Windows. It adds the `errno` entrypoint to the Windows target.
A mistake in the overlay build doc is also fixed.
Tests still cannot be built on Windows because of the lack of osutils.
These were using ” ("right double quotation mark") instead of
the usual ". This means CMake took the value literally and you
got:
```
CMake Error at CMakeLists.txt:139 (MESSAGE):
”libc” isn't a known project:
bolt;clang;clang-tools-extra;compiler-rt;cross-project-tests;libc;libclc;lld;lldb;mlir;openmp;polly;pstl;flang.
Did you mean to enable it as a runtime in LLVM_ENABLE_RUNTIMES?
```
`="libc"` works fine.
Reviewed By: lntue
Differential Revision: https://reviews.llvm.org/D143254
