[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, 3 insertions, 0 deletions

diff --git a/include/clang/ASTMatchers/ASTMatchers.h b/include/clang/ASTMatchers/ASTMatchers.h
index 8bd4429f0b..57dff7993d 100644
--- a/include/clang/ASTMatchers/ASTMatchers.h
+++ b/include/clang/ASTMatchers/ASTMatchers.h
@@ -6171,6 +6171,9 @@ AST_MATCHER(CXXConstructorDecl, isDelegatingConstructor) {
 AST_POLYMORPHIC_MATCHER(isExplicit,
                         AST_POLYMORPHIC_SUPPORTED_TYPES(CXXConstructorDecl,
                                                         CXXConversionDecl)) {
+  // FIXME : it's not clear whether this should match a dependent
+  //         explicit(....). this matcher should also be able to match
+  //         CXXDeductionGuideDecl with explicit specifier.
   return Node.isExplicit();
 }