[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, 1 insertions, 1 deletions

diff --git a/test/CXX/temp/temp.deduct.guide/p1.cpp b/test/CXX/temp/temp.deduct.guide/p1.cpp
index 8bb9da8a97..86aa29dc34 100644
--- a/test/CXX/temp/temp.deduct.guide/p1.cpp
+++ b/test/CXX/temp/temp.deduct.guide/p1.cpp
@@ -71,7 +71,7 @@ extern A(int(&)[26]) -> A<int>;
 #endif
 mutable A(int(&)[27]) -> A<int>; // expected-error-re {{{{'mutable' cannot be applied to|illegal storage class on}} function}}
 virtual A(int(&)[28]) -> A<int>; // expected-error {{'virtual' can only appear on non-static member functions}}
-const A(int(&)[28]) -> A<int>; // expected-error {{deduction guide cannot be declared 'const'}}
+const A(int(&)[31]) -> A<int>; // expected-error {{deduction guide cannot be declared 'const'}}
 
 const volatile static constexpr inline A(int(&)[29]) -> A<int>; // expected-error {{deduction guide cannot be declared 'static inline constexpr const volatile'}}