AppVeyor: Build with clang (incl. LLVM)

When experimenting with the MinGW-w64 based libs, there was a problem -
an undefined reference to function `_(_)chkstk`, emitted by LLVM for
stack probing (e.g., after an `alloca`) and not provided by MinGW-w64.

Initially, I used the symbol exported by ntdll.dll; normal user apps
aren't supposed to depend on that system DLL though, and it also led to
duplicate symbols for 32-bit code.

After some digging, I found that LLVM's builtins compiler-rt library
contains 2 implementations (1 MS-compatible, 1 for MinGW/Cygwin, both
with different names than the MS one). Our library didn't contain it
though, so I digged deeper.

I found the culprit in the builtins CMake script; when using an MSVC
toolchain, the builtins lib on Windows only contains very few C files
and none of the various ASM ones, because they use AT&T syntax.

So I decided to try building LLVM & compiler-rt with clang instead. That
works fine, but I had to fix the CMake script, as the ASM files are
excluded for MS-*compatible* toolchains, incl. clang. So I forked
compiler-rt to fix this and some more annoyances.

When compiling LLVM with clang-cl, the LLVM C++ compile flags include
some clang-specific ones, so building LDC with MSVC didn't work. While
that can be fixed at some point (stripping the clang-specific ones), I
decided to build LDC (and the C/ASM files in our default libs) with
clang too. That required some adaptations, as it started out with > 5k
warnings. ;)

So this commit switches to clang for compiling C/C++/ASM, but still uses
Microsoft's linker.

Author: kinke

appveyor.yml

@@ -87,7 +87,7 @@ install:
           #echo 'Using LLVM with enabled assertions'
           #$assertsSuffix = '-withAsserts'
         }
-        appveyor DownloadFile "https://github.com/ldc-developers/llvm/releases/download/ldc-v$Env:LLVM_VERSION/llvm-$Env:LLVM_VERSION-windows-$Env:APPVEYOR_JOB_ARCH$assertsSuffix.7z" -FileName llvm.7z
+        appveyor DownloadFile "https://github.com/ldc-developers/llvm/releases/download/ldc-v$Env:LLVM_VERSION/llvm-$Env:LLVM_VERSION-windows-$Env:APPVEYOR_JOB_ARCH$assertsSuffix-clang.7z" -FileName llvm.7z
   - md llvm
   - cd llvm
   - 7z x ..\llvm.7z > nul
@@ -98,6 +98,8 @@ install:
   - set PATH=%CD%\llvm\bin;%CD%\ninja;%CD%\make;C:\Program Files\Git\usr\bin;%PATH%
   - if "%APPVEYOR_BUILD_WORKER_IMAGE:~-4%" == "2017" call "C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\Common7\Tools\VsDevCmd.bat" -arch=%APPVEYOR_JOB_ARCH%
   - if "%APPVEYOR_BUILD_WORKER_IMAGE:~-4%" == "2015" call "C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\vcvarsall.bat" %APPVEYOR_JOB_ARCH%
+  # Let CMake configure 64-bit clang-cl for 32-bit code emission
+  - if "%APPVEYOR_JOB_ARCH%" == "x86" ( set "CFLAGS=-m32" && set "CXXFLAGS=-m32" && set "ASMFLAGS=-m32" )
   # Print environment info
   - set
   - msbuild /version
@@ -113,13 +115,13 @@ build_script:
   # Build bootstrap LDC
   - md bootstrap
   - cd bootstrap
-  - cmake -G Ninja -DCMAKE_BUILD_TYPE=Release -DLLVM_ROOT_DIR=c:/projects/llvm ..\ldc
+  - cmake -G Ninja -DCMAKE_C_COMPILER:PATH=clang-cl.exe -DCMAKE_CXX_COMPILER:PATH=clang-cl.exe -DCMAKE_BUILD_TYPE=Release -DLLVM_ROOT_DIR=c:/projects/llvm ..\ldc
   - ninja -j2 all
   - cd ..
   # Build LDC and stdlib unittest runners
   - md ninja-ldc
   - cd ninja-ldc
-  - cmake -G Ninja -DCMAKE_BUILD_TYPE=Release %EXTRA_CMAKE_FLAGS% -DCMAKE_INSTALL_PREFIX=c:\projects\ldc2-%APPVEYOR_JOB_ARCH% -DINCLUDE_INSTALL_DIR=c:/projects/ldc2-%APPVEYOR_JOB_ARCH%/import -DLLVM_ROOT_DIR=c:/projects/llvm -DD_COMPILER=c:\projects\bootstrap\bin\ldmd2.exe ..\ldc
+  - cmake -G Ninja -DCMAKE_C_COMPILER:PATH=clang-cl.exe -DCMAKE_CXX_COMPILER:PATH=clang-cl.exe -DCMAKE_BUILD_TYPE=Release %EXTRA_CMAKE_FLAGS% -DCMAKE_INSTALL_PREFIX=c:\projects\ldc2-%APPVEYOR_JOB_ARCH% -DINCLUDE_INSTALL_DIR=c:/projects/ldc2-%APPVEYOR_JOB_ARCH%/import -DLLVM_ROOT_DIR=c:/projects/llvm -DD_COMPILER=c:\projects\bootstrap\bin\ldmd2.exe ..\ldc
   - ninja -j2 all all-test-runners
 
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