DetermineCompiler: Restore identification of MSVC with no INCLUDE dirs

Since commit bd844387df (ROCMClang: Add the ROCm toolkit derived clang
compiler to CMake, 2020-08-28, v3.21.0-rc1~66^2~6), our generated
compiler id source file contains a preprocessor condition of the form

    #elif ... && __has_include(...)

When MSVC is invoked with no include directories in the `INCLUDE`
environment variable or in `-I` flags, its implementation of
`__has_include` errors out immediately.  This breaks identification of
the compiler.

Work around the problem by adding a fallback attempt to compile the
identification source using a dummy include directory.

Modules/CMakeDetermineCCompiler.cmake

@@ -85,6 +85,10 @@ else()
 
     # ARMClang need target options
     "--target=arm-arm-none-eabi -mcpu=cortex-m3"
+
+    # MSVC needs at least one include directory for __has_include to function,
+    # but custom toolchains may run MSVC with no INCLUDE env var and no -I flags.
+    "-I__does_not_exist__"
     )
 endif()
 if(CMAKE_C_COMPILER_TARGET)

Modules/CMakeDetermineCXXCompiler.cmake

@@ -80,6 +80,10 @@ else()
 
     # ARMClang need target options
     "--target=arm-arm-none-eabi -mcpu=cortex-m3"
+
+    # MSVC needs at least one include directory for __has_include to function,
+    # but custom toolchains may run MSVC with no INCLUDE env var and no -I flags.
+    "-I__does_not_exist__"
     )
 endif()