[clang][timers][modules] Fix a timer being started when it's running

`ASTReader::FinishedDeserializing()` calls
`adjustDeductedFunctionResultType(...)` [0], which in turn calls
`FunctionDecl::getMostRecentDecl()`[1]. In modules builds,
`getMostRecentDecl()` may reach out to the `ASTReader` and start
deserializing again. Starting deserialization starts `ReadTimer`;
however, `FinishedDeserializing()` doesn't call `stopTimer()` until
after it's call to `adjustDeductedFunctionResultType(...)` [2]. As a
result, we hit an assert checking that we don't start an already started
timer [3]. To fix this, we simply don't start the timer if it's already
running.

Unfortunately I don't have a test case for this yet as modules builds
are notoriously difficult to reduce.

[0]: https://github.com/llvm/llvm-project/blob/4d2288d31866013bd361800df4746fbc2fcf742a/clang/lib/Serialization/ASTReader.cpp#L11053
[1]: https://github.com/llvm/llvm-project/blob/4d2288d31866013bd361800df4746fbc2fcf742a/clang/lib/AST/ASTContext.cpp#L3804
[2]: https://github.com/llvm/llvm-project/blob/4d2288d31866013bd361800df4746fbc2fcf742a/clang/lib/Serialization/ASTReader.cpp#L11065-L11066
[3]: https://github.com/llvm/llvm-project/blob/4d2288d31866013bd361800df4746fbc2fcf742a/llvm/lib/Support/Timer.cpp#L150

Author: alanzhao1

clang/lib/Serialization/ASTReader.cpp

@@ -11003,7 +11003,8 @@ void ASTReader::diagnoseOdrViolations() {
 }
 
 void ASTReader::StartedDeserializing() {
-  if (++NumCurrentElementsDeserializing == 1 && ReadTimer.get())
+  if (Timer *T = ReadTimer.get();
+      ++NumCurrentElementsDeserializing == 1 && T && !T->isRunning())
     ReadTimer->startTimer();
 }