HLSL: allow promotion from 1-vector types to scalars, e.g, float<-float1

Previously, an error was thrown when assigning a float1 to a scalar float,
or similar for other basic types.  This allows that.

Also, this allows calling functions accepting scalars with float1 params,
so for example sin(float1) will work.  This is a minor change in
HlslParseContext::findFunction().

gtests/Hlsl.FromFile.cpp

@@ -157,6 +157,7 @@ INSTANTIATE_TEST_CASE_P(
         {"hlsl.pp.line.frag", "main"},
         {"hlsl.precise.frag", "main"},
         {"hlsl.promote.binary.frag", "main"},
+        {"hlsl.promote.vec1.frag", "main"},
         {"hlsl.promotions.frag", "main"},
         {"hlsl.rw.atomics.frag", "main"},
         {"hlsl.rw.bracket.frag", "main"},