[AA] Take account of C++23's stricter rules for forward declarations (NFC) (llvm#109416)

C++23 has stricter rules for forward declarations around
std::unique_ptr, this means that the inline declaration of the
constructor was failing under clang in C++23 mode, switching to an
out-of-line definition of the constructor fixes this.

This was fairly major impact as it blocked inclusion of a lot of headers
under clang in C++23 mode.

Fixes llvm#106597.

Author: aDifferentJT

llvm/include/llvm/Analysis/AliasAnalysis.h

@@ -315,7 +315,7 @@ class AAResults {
 public:
   // Make these results default constructable and movable. We have to spell
   // these out because MSVC won't synthesize them.
-  AAResults(const TargetLibraryInfo &TLI) : TLI(TLI) {}
+  AAResults(const TargetLibraryInfo &TLI);
   AAResults(AAResults &&Arg);
   ~AAResults();
 

llvm/lib/Analysis/AliasAnalysis.cpp

@@ -73,6 +73,8 @@ static cl::opt<bool> EnableAATrace("aa-trace", cl::Hidden, cl::init(false));
 static const bool EnableAATrace = false;
 #endif
 
+AAResults::AAResults(const TargetLibraryInfo &TLI) : TLI(TLI) {}
+
 AAResults::AAResults(AAResults &&Arg)
     : TLI(Arg.TLI), AAs(std::move(Arg.AAs)), AADeps(std::move(Arg.AADeps)) {}