[Attr] Move ParsedTargetAttr out of the TargetAttr class

Need to forward declare it in ASTContext.h for D68627, so it can't be a nested struct.

Differential Revision: https://reviews.llvm.org/D71159
diff --git a/clang/lib/CodeGen/TargetInfo.cpp b/clang/lib/CodeGen/TargetInfo.cpp
index 97bea01..1a78998 100644
--- a/clang/lib/CodeGen/TargetInfo.cpp
+++ b/clang/lib/CodeGen/TargetInfo.cpp
@@ -5061,7 +5061,7 @@
     CodeGenOptions::SignReturnAddressKeyValue Key = CGM.getCodeGenOpts().getSignReturnAddressKey();
     bool BranchTargetEnforcement = CGM.getCodeGenOpts().BranchTargetEnforcement;
     if (const auto *TA = FD->getAttr<TargetAttr>()) {
-      TargetAttr::ParsedTargetAttr Attr = TA->parse();
+      ParsedTargetAttr Attr = TA->parse();
       if (!Attr.BranchProtection.empty()) {
         TargetInfo::BranchProtectionInfo BPI;
         StringRef Error;