Allow list initializer as function argument.

### Problem

Initializer lists are not allowed as function arguments, would be nice if they were, and would possibly simplify some of the MaterialX code base for OSL code generation.

```
struct vector2
{
float x;
float y;
};

void NG_switch_vector2I(vector2 in1, vector2 in2, vector2 in3, output vector2 out)
{
out = in1;
}

shader test_shader
(
vector2 input = {1,2}, // this works
output float out = 0
)
{
vector2 outVec;
vector2 in1 = {0,1}; // this works

NG_switch_vector2I(
in1, // this works
vector2(0.000000, 0.000000), // this works
{0.000000, 0.000000}, // this doesn't work
outVec);

out = outVec.x;
}
```

**Expected behavior:**
I expected this to compile just fine...

**Actual behavior:**
But it didn't... I get the following compile error.
```
> oslc test.osl
test.osl:24: error: Cannot construct type 'unknown'
FAILED test.osl
```

### Steps to Reproduce

1. Compile the shader code above
2. Get compile error

### Versions

* OSL branch/version: 1.13.8.0
* OS: MacOS Sequoia
* C++ compiler:
* LLVM version: 17.0.6
* OIIO version: 2.5.9.0

The reproducer is in test.osl and is compiled through oslc; start by running the reported command to confirm the diagnostic for the braced argument. Trace how the compiler handles the working initializer-list cases versus the function-call argument, then confirm that the example compiles successfully with the braced vector argument.

Written by the indexing model from the issue text.