[NFC] Add Extension Lookup to AArch64TargetParser

Currently, an extension cannot be found using the ExtID. To address this,
the function `lookupExtensionByID` has been added to the `ExtensionSet`
Class so it can be targeted from outside the function.

This will allow for the Extensions Information to be searched for and
stored externally to the ExtensionSet Class. This enables being able to
search for if Architecture Features have been enabled by the user in the
command line.

Author: Stylie777

llvm/include/llvm/TargetParser/AArch64TargetParser.h

@@ -799,6 +799,8 @@ inline constexpr Alias CpuAliases[] = {{"cobalt-100", "neoverse-n2"},
 
 inline constexpr Alias ExtAliases[] = {{"rdma", "rdm"}};
 
+const ExtensionInfo &getExtensionByID(ArchExtKind(ExtID));
+
 bool getExtensionFeatures(
     const AArch64::ExtensionBitset &Extensions,
     std::vector<StringRef> &Features);

llvm/lib/TargetParser/AArch64TargetParser.cpp

@@ -280,3 +280,8 @@ bool AArch64::ExtensionSet::parseModifier(StringRef Modifier) {
   }
   return false;
 }
+
+const AArch64::ExtensionInfo &
+AArch64::getExtensionByID(AArch64::ArchExtKind ExtID) {
+  return lookupExtensionByID(ExtID);
+}