[Hexagon] Tweak _MSC_VER workaround version

My bot runs VS 2019, but it could not compile this code.

Message:
[55/2465] Building CXX object lib\Target\Hexagon\CMakeFiles\LLVMHexagonCodeGen.dir\HexagonVectorCombine.cpp.obj
FAILED: lib/Target/Hexagon/CMakeFiles/LLVMHexagonCodeGen.dir/HexagonVectorCombine.cpp.obj
...
C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\VC\Tools\MSVC\14.23.28105\include\map(71): error C2976: 'std::map': too few template arguments
C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\VC\Tools\MSVC\14.23.28105\include\map(71): note: see declaration of 'std::map'

The version in the path, 14.23, corresponds to _MSC_VER 1923, so raise
the version floor to 1924.

I have not tested with versions between 1924 and 1928 (latest), but the
latest works with the variadic version.