[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, 6 insertions, 2 deletions

diff --git a/test/SemaCXX/explicit.cpp b/test/SemaCXX/explicit.cpp
index a3902e5d10..bd1199f23c 100644
--- a/test/SemaCXX/explicit.cpp
+++ b/test/SemaCXX/explicit.cpp
@@ -1,4 +1,6 @@
 // RUN: %clang_cc1 -fsyntax-only -verify -std=c++11 %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++2a %s
+
 namespace Constructor {
 struct A {
   A(int);
@@ -183,7 +185,8 @@ namespace Conversion {
     const int &copyList7 = {b};
     const int &copyList8 = {n}; // expected-error {{no viable conversion}}
   }
-  
+
+#if __cplusplus < 201707L
   void testNew()
   {
     // 5.3.4p6:
@@ -200,7 +203,8 @@ namespace Conversion {
     new int[i];
     new int[ni]; // expected-error {{array size expression of type 'NotInt' requires explicit conversion to type 'int'}}
   }
-  
+#endif
+
   void testDelete()
   {
     // 5.3.5pp2: