Fix CI: json_view tests without exceptions, on clang 3.6, and single header

- ci_test_noexceptions: the helpers that compare the exceptions of
  json_document::parse() and json::parse() catch them outside a
  CHECK_THROWS, so with JSON_NOEXCEPTION the first parse error aborted
  the test. Compile those comparisons only with exceptions, as
  unit-class_parser.cpp does.
- ci_test_gcc: -Werror=unused-result for CHECK_THROWS_AS(json_document::
  parse(...)); assign the result to a dummy document.
- ci_test_compilers_clang (3.6): `const json_view invalid;` needs a
  user-provided default constructor there (CWG 253); value-initialize it.
- ci_test_single_header: json_view.hpp now exists as a single header and
  contains the internal view headers, so unit-json_view_builder.cpp
  includes it instead of the detail headers in that mode, and the test
  is built again with the single header.
- Regenerate single_include/nlohmann/json_view.hpp for the builder change
  merged from json-view/08-view-builder.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

single_include/nlohmann/json_view.hpp

@@ -955,6 +955,14 @@ class builder
         return w;
     }
 
+    /// a compile-time option as a runtime condition: testing the template
+    /// argument directly makes a condition like `TrailingCommas && c == ']'`
+    /// constant when the option is off, which MSVC reports as C4127
+    static NLOHMANN_VIEW_ALWAYS_INLINE bool enabled(bool option) noexcept
+    {
+        return option;
+    }
+
     /// The parse state and the parser proper. The cursor is a local object of
     /// run() whose address never escapes (everything it calls out of line is a
     /// member of the builder and gets the positions it needs), so that the
@@ -1084,7 +1092,7 @@ arr_next:
                 {
                     return false;
                 }
-                if (TrailingCommas && cur() == ']')
+                if (enabled(TrailingCommas) && cur() == ']')
                 {
                     ++p;
                     goto close_container;
@@ -1151,7 +1159,7 @@ obj_next:
                 {
                     return false;
                 }
-                if (TrailingCommas && cur() == '}')
+                if (enabled(TrailingCommas) && cur() == '}')
                 {
                     ++p;
                     goto close_container;
@@ -1293,7 +1301,7 @@ indent_done:
                 {
                     ++p;
                 }
-                if (Comments && cur() == '/')
+                if (enabled(Comments) && cur() == '/')
                 {
                     const unsigned char* const q = cold.comment(p);
                     if (q == nullptr)