[Clang][Parse] Fix ambiguity with nested-name-specifiers that may declarative (#96364)

Consider the following:
```
template<typename T>
struct A { };

template<typename T>
int A<T>::B::* f(); // error: no member named 'B' in 'A<T>'
```

Although this is clearly valid, clang rejects it because the
_nested-name-specifier_ `A<T>::` is parsed as-if it was declarative,
meaning, we parse it as-if it was the _nested-name-specifier_ in a
redeclaration/specialization. However, we don't (and can't) know whether
the _nested-name-specifier_ is declarative until we see the '`*`' token,
but at that point we have already complained that `A` has no member
named `B`! This patch addresses this bug by adding support for _fully_
unannotated _and_ unbounded tentative parsing, which allows for us to
parse past tokens without having to cache them until we reach a point
where we can guarantee to be past the construct we are disambiguating.

I don't know where the approach taken here is ideal -- alternatives are
welcome. However, the performance impact (as measured by
llvm-compile-time-tracker (https://llvm-compile-time-tracker.com/?config=Overview&stat=instructions%3Au&remote=sdkrystian)
is quite minimal (0.09%, which I plan to further improve).

1 files changed, 1 insertions, 1 deletions

diff --git a/clang/lib/Lex/Preprocessor.cpp b/clang/lib/Lex/Preprocessor.cpp
index 63e27e6..f0b4593 100644
--- a/clang/lib/Lex/Preprocessor.cpp
+++ b/clang/lib/Lex/Preprocessor.cpp
@@ -170,7 +170,7 @@ Preprocessor::Preprocessor(std::shared_ptr<PreprocessorOptions> PPOpts,
 }
 
 Preprocessor::~Preprocessor() {
-  assert(BacktrackPositions.empty() && "EnableBacktrack/Backtrack imbalance!");
+  assert(!isBacktrackEnabled() && "EnableBacktrack/Backtrack imbalance!");
 
   IncludeMacroStack.clear();