Enable constexpr on Visual Studio 2015, add support for two equivalent attributes

Enable constexpr on Visual Studio 2015, add support for two equivalent attributes

Patch by Alexander Riccio

This patch enables `constexpr` on Visual Studio 2015 by adding `||
LLVM_MSC_PREREQ(1900)` to the preprocessor `#if` statement. Since VS2013
doesn't support `constexpr`, that's purposely excluded. The
`LLVM_CONSTEXPR` macro is used in ~25 places.

I also added the MSVC/SAL equivalent of:

  - `__attribute__((__warn_unused_result__))` as an
    `LLVM_ATTRIBUTE_UNUSED_RESULT` definition

  - `__attribute__((returns_nonnull))` as an
    `LLVM_ATTRIBUTE_RETURNS_NONNULL` definition

...in case anybody ever decides to run `/analyze` on LLVM (probably
myself, if anybody)

Reviewers: rnk, aaron.ballman

Differential Revision: http://reviews.llvm.org/D16751

llvm-svn: 260410