Fix standard integer definitions for MSVC in DataTypes.h

Previously, DataTypes.h would #define a variety of symbols any time
they weren't already defined.  However, some versions of Visual
Studio do provide the appropriate headers, so if those headers are
included after DataTypes.h, it can lead to macro redefinition
warnings.

The fix is to include the appropriate headers if they exist, and
only #define the symbols if the required header does not exist.

Patch by Zachary Turner!

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The big change here is that we no longer have our own stdint.h
typedefs because now all supported toolchains have stdint.h.
Hooray!

llvm-svn: 209760