[AArch64][TargetParser] Add field for FEAT_* feature names to ExtensionInfo

This introduces a new textual field for the Architecture Feature names
to the AArch64 extensions in TableGen and TargetParser. So far, any
cross references between the extensions in the implementation and the
ones from the ISA has been covered only by the free text in ther
'Description' field. The new field adds a reliable and explicit mapping
between them, and enables enhancements in testability that will be
implemented in an upcoming patch.

Author: pratlucas

clang/test/Driver/print-supported-extensions.c

@@ -4,8 +4,8 @@
 // RUN: %if aarch64-registered-target %{ %clang --target=aarch64-linux-gnu \
 // RUN:   --print-supported-extensions 2>&1 | FileCheck %s --check-prefix AARCH64 %}
 // AARCH64: All available -march extensions for AArch64
-// AARCH64:     Name                Description
-// AARCH64:     aes                 Enable AES support (FEAT_AES, FEAT_PMULL)
+// AARCH64:     Name                Architecture Feature(s)                                Description
+// AARCH64:     aes                 FEAT_AES, FEAT_PMULL                                   Enable AES support
 
 // RUN: %if riscv-registered-target %{ %clang --target=riscv64-linux-gnu \
 // RUN:   --print-supported-extensions 2>&1 | FileCheck %s --check-prefix RISCV %}

llvm/include/llvm/TargetParser/AArch64TargetParser.h

@@ -118,6 +118,7 @@ struct ExtensionInfo {
   std::optional<StringRef> Alias; // An alias for this extension, if one exists.
   ArchExtKind ID;                 // Corresponding to the ArchExtKind, this
                                   // extensions representation in the bitfield.
+  StringRef ArchFeatureName;      // The feature name defined by the Architecture
   StringRef Description;          // The textual description of the extension
   StringRef TargetFeature;        // -target-feature/-mattr enable string, e.g. "+spe"
   StringRef NegTargetFeature;     // -target-feature/-mattr disable string, e.g. "-spe"