[Clang][Sema]Choose a better candidate in overload function call if there is a...

[Clang][Sema]Choose a better candidate in overload function call if there is a compatible vector conversion instead of ambiguous call error

There are 2 function variations with vector type parameter. When we call them with argument of different vector type we would prefer to 
choose the variation with implicit argument conversion of compatible vector type instead of incompatible vector type. For example,

typedef float __v4sf __attribute__((__vector_size__(16)));
void f(vector float);
void f(vector signed int);

int main {
   __v4sf a;
   f(a);
}

Here, we'd like to choose f(vector float) but not report an ambiguous call error.

Differential revision: https://reviews.llvm.org/D53417

llvm-svn: 347019