[AsmParser] Unify parsing of attributes

Continuing on from D105780, this should be the last major bit of
attribute cleanup. Currently, LLParser implements attribute parsing
for functions, parameters and returns separately, enumerating all
supported (and unsupported) attributes each time. This patch
extracts the common parsing logic, and performs a check afterwards
whether the attribute is valid in the given position. Parameters
and returns are handled together, while function attributes need
slightly different logic to support attribute groups.

Differential Revision: https://reviews.llvm.org/D105938

llvm/include/llvm/AsmParser/LLParser.h

@@ -258,8 +258,15 @@ namespace llvm {
       return parseOptionalAddrSpace(
           AddrSpace, M->getDataLayout().getProgramAddressSpace());
     };
-    bool parseOptionalParamAttrs(AttrBuilder &B);
-    bool parseOptionalReturnAttrs(AttrBuilder &B);
+    bool parseEnumAttribute(Attribute::AttrKind Attr, AttrBuilder &B,
+                            bool InAttrGroup);
+    bool parseOptionalParamOrReturnAttrs(AttrBuilder &B, bool IsParam);
+    bool parseOptionalParamAttrs(AttrBuilder &B) {
+      return parseOptionalParamOrReturnAttrs(B, true);
+    }
+    bool parseOptionalReturnAttrs(AttrBuilder &B) {
+      return parseOptionalParamOrReturnAttrs(B, false);
+    }
     bool parseOptionalLinkage(unsigned &Res, bool &HasLinkage,
                               unsigned &Visibility, unsigned &DLLStorageClass,
                               bool &DSOLocal);