[llvm] [SYCL] Add libsycl, a SYCL RT library implementation project (PR #144372)

[Tom Honermann via llvm-commits]

================
@@ -0,0 +1,115 @@
+#===============================================================================
+# Setup Project
+#===============================================================================
+cmake_minimum_required(VERSION 3.20.0)
+
+set(LLVM_SUBPROJECT_TITLE "libsycl")
+
+set(LIBSYCL_SOURCE_DIR  ${CMAKE_CURRENT_SOURCE_DIR})
+set(LIBSYCL_BINARY_DIR  ${CMAKE_CURRENT_BINARY_DIR})
+
+set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD_REQUIRED YES)
+set(CMAKE_CXX_EXTENSIONS OFF)
+
+#===============================================================================
+# Limitations
+#===============================================================================
+
+if (CMAKE_SYSTEM_NAME STREQUAL Windows AND NOT MSVC)
+# Build with other compilers is not configured, not guaranteed and not tested.
+    message( FATAL_ERROR
+      "On Windows libsycl supports compiler which is some version of Microsoft"
+      " Visual C++ or another compiler simulating the Visual C++ cl"
+      " command-line syntax" )
----------------
tahonermann wrote:

```suggestion
    message(FATAL_ERROR
      "When compiling for Windows, libsycl requires a"
      " version of Microsoft Visual C++ or another compiler"
      " that uses the Visual C++ cl command-line syntax.")
```

https://github.com/llvm/llvm-project/pull/144372