[Clang] Wide delimiters ('{{{') for expect strings

Prior to this commit, it was impossible to use the simple string
matching directives to look for most content that contains `{{`, such
as:

```
// expected-note {{my_struct{{1}, 2}}}
```

Which would parse like so:

```
             "nested" brace v
// expected-note {{my_struct{{1}, 2}}}
          closes the nested brace  ^ |
                            trailing }
```

And the frontend would complain 'cannot find end ('}}') of expected'.

At this snapshot, VerifyDiagnosticConsumer's parser now counts the
opening braces and looks for a matching length of closing sigils,
allowing the above to be written as:

```
// expected-note {{{my_struct{{1}, 2}}}}
   opening brace |-|                 |-|
  closing brace is '}}}', found here ^
```

Author: sethp

clang/include/clang/Basic/DiagnosticFrontendKinds.td

@@ -166,7 +166,7 @@ def err_verify_no_such_marker : Error<
 def err_verify_missing_start : Error<
     "cannot find start ('{{') of expected %0">;
 def err_verify_missing_end : Error<
-    "cannot find end ('}}') of expected %0">;
+    "cannot find end ('%1') of expected %0">;
 def err_verify_invalid_content : Error<
     "invalid expected %0: %1">;
 def err_verify_missing_regex : Error<

clang/lib/Frontend/VerifyDiagnosticConsumer.cpp

@@ -612,12 +612,19 @@ static bool ParseDirective(StringRef S, ExpectedData *ED, SourceManager &SM,
                    diag::err_verify_missing_start) << KindStr;
       continue;
     }
+    llvm::SmallString<8> CloseBrace("}}");
+    const char *const DelimBegin = PH.C;
     PH.Advance();
+    // Count the number of opening braces for `string` kinds
+    for (; !D.RegexKind && PH.Next("{"); PH.Advance())
+      CloseBrace += '}';
     const char* const ContentBegin = PH.C; // mark content begin
-    // Search for token: }}
-    if (!PH.SearchClosingBrace("{{", "}}")) {
-      Diags.Report(Pos.getLocWithOffset(PH.C-PH.Begin),
-                   diag::err_verify_missing_end) << KindStr;
+    // Search for closing brace
+    StringRef OpenBrace(DelimBegin, ContentBegin - DelimBegin);
+    if (!PH.SearchClosingBrace(OpenBrace, CloseBrace)) {
+      Diags.Report(Pos.getLocWithOffset(PH.C - PH.Begin),
+                   diag::err_verify_missing_end)
+          << KindStr << CloseBrace;
       continue;
     }
     const char* const ContentEnd = PH.P; // mark content end

clang/test/SemaCXX/static-assert-diagnostics.cpp

@@ -20,8 +20,7 @@ struct _arr {
   }
 };
 
-// output: '{{2, 3, 4}} == {0, 3, 4}'  (the `{{` breaks VerifyDiagnosticConsumer::ParseDirective)
-// expected-note@+1 {{evaluates to}}
+// expected-note@+1 {{{evaluates to '{{2, 3, 4}} == {0, 3, 4}'}}}
 static_assert(_arr{2, 3, 4} == a0.b); // expected-error {{failed}}
 
 struct B {
@@ -41,8 +40,7 @@ struct C: A, B {
 
 constexpr auto cc = C{A{1, {2, 3, 4}, 5}, B{7, 6}, C::E1};
 
-// actually '{{1, {2, 3, 4}, 5}, {7, 6}, 0} == {{0, {0, 3, 4}, 5}, {5, 0}, 1}'  (the `{{` breaks VerifyDiagnosticConsumer::ParseDirective)
-// expected-note@+1 {{evaluates to}}
+// expected-note@+1 {{{evaluates to '{{1, {2, 3, 4}, 5}, {7, 6}, 0} == {{0, {0, 3, 4}, 5}, {5, 0}, 1}'}}}
 static_assert(cc == C{a0, {5}, C::E2}); // expected-error {{failed}}
 
 // this little guy? oh, I wouldn't worry about this little guy
@@ -83,8 +81,7 @@ struct V {
 };
 static_assert(V{1, 2, 3, 4} == V{1, 2, 3, 4});
 
-// '{{1, 2, 3, 4}} == {{1, 2, 0, 4}}'
-// expected-note@+1 {{evaluates to}}
+// expected-note@+1 {{{evaluates to '{{1, 2, 3, 4}} == {{1, 2, 0, 4}}'}}}
 static_assert(V{1, 2, 3, 4} == V{1, 2, 0, 4}); // expected-error {{failed}}
 
 constexpr auto v = (v4si){1, 2, 3, 4};
@@ -105,6 +102,5 @@ struct BV {
   }
 };
 
-// '{{false, true, false, false, false, false, false, false}} == {{true, false, false, false, false, false, false, false}}'
-// expected-note@+1 {{evaluates to}}
+// expected-note@+1 {{{evaluates to '{{false, true, false, false, false, false, false, false}} == {{true, false, false, false, false, false, false, false}}'}}}
 static_assert(BV{{0, 1}} == BV{{1, 0}}); // expected-error {{failed}}