Revert r359949 "[clang] adding explicit(bool) from c++2a"

This caused Clang to start erroring on the following:

  struct S {
    template <typename = int> explicit S();
  };

  struct T : S {};

  struct U : T {
    U();
  };
  U::U() {}

  $ clang -c /tmp/x.cc
  /tmp/x.cc:10:4: error: call to implicitly-deleted default constructor of 'T'
  U::U() {}
     ^
  /tmp/x.cc:5:12: note: default constructor of 'T' is implicitly deleted
    because base class 'S' has no default constructor
  struct T : S {};
             ^
  1 error generated.

See discussion on the cfe-commits email thread.

This also reverts the follow-ups r359966 and r359968.

> this patch adds support for the explicit bool specifier.
>
> Changes:
> - The parsing for the explicit(bool) specifier was added in ParseDecl.cpp.
> - The storage of the explicit specifier was changed. the explicit specifier was stored as a boolean value in the FunctionDeclBitfields and in the DeclSpec class. now it is stored as a PointerIntPair<Expr*, 2> with a flag and a potential expression in CXXConstructorDecl, CXXDeductionGuideDecl, CXXConversionDecl and in the DeclSpec class.
> - Following the AST change, Serialization, ASTMatchers, ASTComparator and ASTPrinter were adapted.
> - Template instantiation was adapted to instantiate the potential expressions of the explicit(bool) specifier When instantiating their associated declaration.
> - The Add*Candidate functions were adapted, they now take a Boolean indicating if the context allowing explicit constructor or conversion function and this boolean is used to remove invalid overloads that required template instantiation to be detected.
> - Test for Semantic and Serialization were added.
>
> This patch is not yet complete. I still need to check that interaction with CTAD and deduction guides is correct. and add more tests for AST operations. But I wanted first feedback.
> Perhaps this patch should be spited in smaller patches, but making each patch testable as a standalone may be tricky.
>
> Patch by Tyker
>
> Differential Revision: https://reviews.llvm.org/D60934

llvm-svn: 360024
diff --git a/clang/lib/AST/ASTImporter.cpp b/clang/lib/AST/ASTImporter.cpp
index 1ab99be..1ac4870 100644
--- a/clang/lib/AST/ASTImporter.cpp
+++ b/clang/lib/AST/ASTImporter.cpp
@@ -3055,20 +3055,11 @@
   // Create the imported function.
   FunctionDecl *ToFunction = nullptr;
   if (auto *FromConstructor = dyn_cast<CXXConstructorDecl>(D)) {
-    Expr *ExplicitExpr = nullptr;
-    if (FromConstructor->getExplicitSpecifier().getExpr()) {
-      auto Imp = importSeq(FromConstructor->getExplicitSpecifier().getExpr());
-      if (!Imp)
-        return Imp.takeError();
-      std::tie(ExplicitExpr) = *Imp;
-    }
     if (GetImportedOrCreateDecl<CXXConstructorDecl>(
-            ToFunction, D, Importer.getToContext(), cast<CXXRecordDecl>(DC),
-            ToInnerLocStart, NameInfo, T, TInfo,
-            ExplicitSpecifier(
-                ExplicitExpr,
-                FromConstructor->getExplicitSpecifier().getKind()),
-            D->isInlineSpecified(), D->isImplicit(), D->isConstexpr()))
+        ToFunction, D, Importer.getToContext(), cast<CXXRecordDecl>(DC),
+        ToInnerLocStart, NameInfo, T, TInfo,
+        FromConstructor->isExplicit(),
+        D->isInlineSpecified(), D->isImplicit(), D->isConstexpr()))
       return ToFunction;
   } else if (CXXDestructorDecl *FromDtor = dyn_cast<CXXDestructorDecl>(D)) {
 
@@ -3094,19 +3085,10 @@
     ToDtor->setOperatorDelete(ToOperatorDelete, ToThisArg);
   } else if (CXXConversionDecl *FromConversion =
                  dyn_cast<CXXConversionDecl>(D)) {
-    Expr *ExplicitExpr = nullptr;
-    if (FromConversion->getExplicitSpecifier().getExpr()) {
-      auto Imp = importSeq(FromConversion->getExplicitSpecifier().getExpr());
-      if (!Imp)
-        return Imp.takeError();
-      std::tie(ExplicitExpr) = *Imp;
-    }
     if (GetImportedOrCreateDecl<CXXConversionDecl>(
             ToFunction, D, Importer.getToContext(), cast<CXXRecordDecl>(DC),
             ToInnerLocStart, NameInfo, T, TInfo, D->isInlineSpecified(),
-            ExplicitSpecifier(ExplicitExpr,
-                              FromConversion->getExplicitSpecifier().getKind()),
-            D->isConstexpr(), SourceLocation()))
+            FromConversion->isExplicit(), D->isConstexpr(), SourceLocation()))
       return ToFunction;
   } else if (auto *Method = dyn_cast<CXXMethodDecl>(D)) {
     if (GetImportedOrCreateDecl<CXXMethodDecl>(