Fix corefx to build on clang 10

Clang 10 adds/enables new warnings, some of which is affecting the
corefx code. Disable them.

This is a "backport" of dotnet/runtime#33734 to corefx.

After this commit, I can build all of corefx with Clang 10.

src/Native/Unix/CMakeLists.txt

@@ -23,6 +23,10 @@ add_compile_options(-I${CMAKE_CURRENT_SOURCE_DIR}/Common)
 add_compile_options(-I${CMAKE_CURRENT_BINARY_DIR}/Common)
 add_compile_options(-g)
 if(CMAKE_C_COMPILER_ID STREQUAL Clang)
+    if (CMAKE_C_COMPILER_VERSION VERSION_GREATER_EQUAL 10)
+        add_compile_options(-Wno-alloca)
+        add_compile_options(-Wno-implicit-int-float-conversion)
+    endif()
     add_compile_options(-Wthread-safety)
     add_compile_options(-Wno-thread-safety-analysis)
 endif()