VS: Fix GenerateDebugInformation values for v140 and v141 toolsets

When VS 2015 was first released, its new v140 toolset came with a
`link.xml` file that changed the `GenerateDebugInformation` boolean
(`false` and `true`) value from earlier toolsets to an enumeration
consisting of the possible values `No`, `Debug`, and `DebugFastLink`.

We first adapted to this in commit v3.4.2~2^2 (VS: Fix VS 2015 .vcxproj
file value for GenerateDebugInformation, 2016-01-08), but that broke
older toolsets that still expected the boolean.  Then commit
v3.6.0-rc1~295^2~1 (VS: Fix VS 2015 .vcxproj debug setting for older
toolsets, 2016-02-24) added a hack to fix up the value based on the
toolset in use.  Several follow-up commits fixed this for more older
toolsets because our flag table was at the time based on the generator
in use rather than the toolset in use.

Since commit v3.8.0-rc1~396^2 (VS: Choose flag map based on the toolset
name, 2016-10-17) we use a flag table based on the toolset, so the fixup
hack should not be needed.  We had to keep it around only due to our
default value for GenerateDebugInformation (`false` or `No`) still being
based on the generator instead of the toolset.

A VS 2015 update was released that changed the v140 toolset `link.xml`
file back to using `false` and `true` for the `GenerateDebugInformation`
enumeration variants previously known as `No` and `Debug`.  In order to
know which pair to use, we need to parse the `link.xml` file for the
current toolset.

Switch back to using `false` and `true` unconditionally in our
`GenerateDebugInformation` flag table entries and default value.  With
that plus the toolset-based flag table, we now get incorrect values for
`GenerateDebugInformation` only when using a v140 toolset from an older
VS 2015 installation.  Detect this case by parsing `link.xml` and add
special logic to convert `false` and `true` to `No` and `Debug` to
satisfy the older toolset specification.

Inspired-by: Ian Hojnicki <nullref@live.com>
Fixes: #17020