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

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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@359949 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 7 insertions, 2 deletions

diff --git a/lib/Sema/SemaLookup.cpp b/lib/Sema/SemaLookup.cpp
index 2b7924d244..a91c20e00a 100644
--- a/lib/Sema/SemaLookup.cpp
+++ b/lib/Sema/SemaLookup.cpp
@@ -3041,10 +3041,15 @@ Sema::SpecialMemberOverloadResult Sema::LookupSpecialMember(CXXRecordDecl *RD,
                            llvm::makeArrayRef(&Arg, NumArgs), OCS, true);
       else if (CtorInfo)
         AddOverloadCandidate(CtorInfo.Constructor, CtorInfo.FoundDecl,
-                             llvm::makeArrayRef(&Arg, NumArgs), OCS, true);
+                             llvm::makeArrayRef(&Arg, NumArgs), OCS,
+                             /*SuppressUserConversions*/ true,
+                             /*PartialOverloading*/ false,
+                             /*AllowExplcit*/ true);
       else
         AddOverloadCandidate(M, Cand, llvm::makeArrayRef(&Arg, NumArgs), OCS,
-                             true);
+                             /*SuppressUserConversions*/ true,
+                             /*PartialOverloading*/ false,
+                             /*AllowExplcit*/ true);
     } else if (FunctionTemplateDecl *Tmpl =
                  dyn_cast<FunctionTemplateDecl>(Cand->getUnderlyingDecl())) {
       if (SM == CXXCopyAssignment || SM == CXXMoveAssignment)