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