Update WindowsBuild.md

Add instructions for building libxml2.  The libxml2 available for Windows is built against MinGW and requires iconv support.  Simply build our own version of libxml2 to avoid the dependency.

Author: compnerd

docs/WindowsBuild.md

@@ -41,6 +41,7 @@ it provides some of the needed headers and libraries.
 1. Clone `apple/swift-corelibs-foundation` into a folder named `swift-corelibs-foundation`
 1. Clone `apple/swift-corelibs-xctest` into a folder name `swift-corelibs-xctest`
 1. Clone `apple/swift-lldb` into a folder named `lldb`
+1. Clone `libxml2` into a folder named `libxml2`
 
 - Currently, other repositories in the Swift project have not been tested and
   may not be supported.
@@ -63,6 +64,7 @@ git clone https://github.com/apple/swift-corelibs-libdispatch
 git clone https://github.com/apple/swift-corelibs-foundation
 git clone https://github.com/apple/swift-corelibs-xctest
 git clone https://github.com/apple/swift-lldb lldb
+git clone https://gitlab.gnome.org/GNOME/libxml2.git libxml2
 ```
 
 ### 3. Acquire ICU
@@ -242,11 +244,19 @@ popd
 cmake --build S:\b\libdispatch
 ```
 
-### 11. Build swift-corelibs-foundation
+### 11. Build libxml2
+
+```cmd
+pushd "S:\libxml2\win32"
+cscript configure.js iconv=no
+nmake /f Makefile.msvc
+popd
+```
+
+### 12. Build swift-corelibs-foundation
 
 To build Foundation you will need builds of:
 
-- `libxml2` (http://xmlsoft.org, download and unzip the Windows prebuilt or build your own)
 - `libcurl` (https://curl.haxx.se, download the source, `cd` into `winbuild`, and run `nmake /f Makefile.vc mode=static VC=15 MACHINE=x64`)
 
 ```cmd
@@ -269,7 +279,7 @@ cmake -G Ninja^
 
 ```
 
-### 12. Build swift-corelibs-xctest
+### 13. Build swift-corelibs-xctest
 
 ```cmd
 mkdir "S:\b\xctest"
@@ -286,7 +296,7 @@ popd
 cmake --build S:\b\xctest
 ```
 
-### 13. Install Swift on Windows
+### 14. Install Swift on Windows
 
 - Run ninja install: