glsl: Split up function matching and call generation a bit more.

glsl: Split up function matching and call generation a bit more.

We used to have one big function, match_signature_by_name, which found
a matching signature, performed out-parameter conversions, and generated
the ir_call.  As the code for matching against built-in functions became
more complicated, I split it internally, creating generate_call().

However, I left the same awkward interface.  This patch splits it into
three functions:
1. match_signature_by_name()

   This now takes a name, a list of parameters, the symbol table, and
   returns an ir_function_signature.  Simple and one purpose: matching.

2. no_matching_function_error()

   Generate the "no matching function" error and list of prototypes.
   This was complex enough that I felt it deserved its own function.

3. generate_call()

   Do the out-parameter conversion and generate the ir_call.  This
   could probably use more splitting.

The caller now has a more natural workflow: find a matching signature,
then either generate an error or a call.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>