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/ASTStructuralEquivalence.cpp b/clang/lib/AST/ASTStructuralEquivalence.cpp
index f3c34d7..71bbd82 100644
--- a/clang/lib/AST/ASTStructuralEquivalence.cpp
+++ b/clang/lib/AST/ASTStructuralEquivalence.cpp
@@ -955,15 +955,13 @@
 
   if (auto *Constructor1 = dyn_cast<CXXConstructorDecl>(Method1)) {
     auto *Constructor2 = cast<CXXConstructorDecl>(Method2);
-    if (!Constructor1->getExplicitSpecifier().isEquivalent(
-            Constructor2->getExplicitSpecifier()))
+    if (Constructor1->isExplicit() != Constructor2->isExplicit())
       return false;
   }
 
   if (auto *Conversion1 = dyn_cast<CXXConversionDecl>(Method1)) {
     auto *Conversion2 = cast<CXXConversionDecl>(Method2);
-    if (!Conversion1->getExplicitSpecifier().isEquivalent(
-            Conversion2->getExplicitSpecifier()))
+    if (Conversion1->isExplicit() != Conversion2->isExplicit())
       return false;
     if (!IsStructurallyEquivalent(Context, Conversion1->getConversionType(),
                                   Conversion2->getConversionType()))