Driver: enhance MSC version compatibility

The version information for Visual Studio is spread over multiple variables.
The newer Windows SDK has started making use of some of the extended versioning
variables that were previously undefined.  Enhance our compatibility definitions
for these cases.

_MSC_VER is defined to be the Major * 100 + Minor.  _MSC_FULL_VER is defined to
be Major * 10000000 + Minor * 100000 + Build.  And _MSC_BUILD is the build
revision of the compiler.

Extend the -fmsc-version option in a compatible manner.  If the value is the
previous form of MMmm, then we assume that the build number is 0.  Otherwise, a
specific build number may be passed by using the form MMmmbbbbb.  Due to
bitwidth limitations of the option, it is currently not possible to define a
revision value.

The version information can be passed as either the decimal encoded value
(_MSC_FULL_VER or _MSC_VER) or as a dot-delimited value.

The change to the TextDiagnostic is to deal with the updated encoding of the
version information.