Declare __builtin_strlen in StringRef.h as constexpr

Otherwise Visual Studio 2017 will complain about
llvm::StringRef::strlen not being constexpr:

  StringRef.h(80): error C3615: constexpr function 'llvm::StringRef::strLen' cannot result in a constant expression
  StringRef.h(84): note: failure was caused by call of undefined function or one not declared 'constexpr'

(cherry picked from commit b3d7d106)