[Sema] Support implicit scalar to vector conversions

This patch teaches clang to perform implicit scalar to vector conversions
when one of the operands of a binary vector expression is a scalar which
can be converted to the element type of the vector without truncation
following GCC's implementation.

If the (constant) scalar is can be casted safely, it is implicitly casted to the
vector elements type and splatted to produce a vector of the same type.

Contributions from: Petar Jovanovic

Reviewers: bruno, vkalintiris

Differential Revision: https://reviews.llvm.org/D25866

llvm-svn: 302935