[TLI] Use AArch64 vector calling convention for ArmPL routines (#135790)

Some ArmPL vector math routines that operate on fixed-width
vector types obey the aarch64_vector_pcs calling convention,
e.g. armpl_vsinq_f64, etc. This patch adds support for
functions in the TLI to specify an optional calling
convention to be added to the declared function. Once this
patch lands I intend to follow up with a vectoriser patch
that adds this optional calling convention when invoking the
vector library variant from the loop vectoriser.

Author: david-arm

llvm/include/llvm/Analysis/TargetLibraryInfo.h

@@ -45,20 +45,23 @@ class VecDesc {
   ElementCount VectorizationFactor;
   bool Masked;
   StringRef VABIPrefix;
+  std::optional<CallingConv::ID> CC;
 
 public:
   VecDesc() = delete;
   VecDesc(StringRef ScalarFnName, StringRef VectorFnName,
-          ElementCount VectorizationFactor, bool Masked, StringRef VABIPrefix)
+          ElementCount VectorizationFactor, bool Masked, StringRef VABIPrefix,
+          std::optional<CallingConv::ID> Conv)
       : ScalarFnName(ScalarFnName), VectorFnName(VectorFnName),
         VectorizationFactor(VectorizationFactor), Masked(Masked),
-        VABIPrefix(VABIPrefix) {}
+        VABIPrefix(VABIPrefix), CC(Conv) {}
 
   StringRef getScalarFnName() const { return ScalarFnName; }
   StringRef getVectorFnName() const { return VectorFnName; }
   ElementCount getVectorizationFactor() const { return VectorizationFactor; }
   bool isMasked() const { return Masked; }
   StringRef getVABIPrefix() const { return VABIPrefix; }
+  std::optional<CallingConv::ID> getCallingConv() const { return CC; }
 
   /// Returns a vector function ABI variant string on the form:
   ///    _ZGV<isa><mask><vlen><vparams>_<scalarname>(<vectorname>)