Thread safety analysis: Peel away NoOp implicit casts in initializers

Summary:
This happens when someone initializes a variable with guaranteed copy
elision and an added const qualifier. Fixes PR43826.

Reviewers: aaron.ballman, rsmith

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D69533
diff --git a/clang/lib/Analysis/ThreadSafety.cpp b/clang/lib/Analysis/ThreadSafety.cpp
index c609543..48f4106 100644
--- a/clang/lib/Analysis/ThreadSafety.cpp
+++ b/clang/lib/Analysis/ThreadSafety.cpp
@@ -2142,6 +2142,9 @@
       // handle constructors that involve temporaries
       if (auto *EWC = dyn_cast<ExprWithCleanups>(E))
         E = EWC->getSubExpr();
+      if (auto *ICE = dyn_cast<ImplicitCastExpr>(E))
+        if (ICE->getCastKind() == CK_NoOp)
+          E = ICE->getSubExpr();
       if (auto *BTE = dyn_cast<CXXBindTemporaryExpr>(E))
         E = BTE->getSubExpr();