clang-format: [JS] handle semis in generic types.

[mprobst]

Summary

TypeScript generic type arguments can contain object (literal) types,
which in turn can contain semicolons:

const x: Array<{a: number; b: string;} = [];

Previously, clang-format would incorrectly categorize the braced list as
a block and terminate the line at the openening {, and then format the
entire expression badly.

With this change, clang-format recognizes < preceding a { as
introducing a type expression. In JS, < comparison with an object
literal can never be true, so the chance of introducing false positives
here is very low.

[djasper]

Looks good.

lib/Format/UnwrappedLineParser.cpp

do {
     do {
       NextTok = Tokens->getNextToken();
       ++ReadTokens;
     } while (NextTok->is(tok::comment));
 
     switch (Tok->Tok.getKind()) {
     case tok::l_brace:
       if (Style.Language == FormatStyle::LK_JavaScript && PrevTok) {
-        if (PrevTok->is(tok::colon))
-          // A colon indicates this code is in a type, or a braced list
-          // following a label in an object literal ({a: {b: 1}}). The code
-          // below could be confused by semicolons between the individual
-          // members in a type member list, which would normally trigger
-          // BK_Block. In both cases, this must be parsed as an inline braced
-          // init.
+        if (PrevTok->isOneOf(tok::colon, tok::less))
+          // A ':' indicates this code is in a type, or a braced list
+          // following a label in an object literal ({a: {b: 1}}).
+          // A '<' could be an object used in a comparison, but that is nonsense
+          // code (can never return true), so more likely it is a generic type
+          // argument (`X<{a: string; b: number}>`).
+          // The code below could be confused by semicolons between the
+          // individual members in a type member list, which would normally
+          // trigger BK_Block. In both cases, this must be parsed as an inline
+          // braced init.
           Tok->BlockKind = BK_BracedInit;
         else if (PrevTok->is(tok::r_paren))
           // `) { }` can only occur in function or method declarations in JS.
           Tok->BlockKind = BK_Block;
       } else {
         Tok->BlockKind = BK_Unknown;
       }
       LBraceStack.push_back(Tok);

unittests/Format/FormatTestJS.cpp

TEST_F(FormatTestJS, TypeAnnotations) {
   verifyFormat("function x(): string {\n  return 'x';\n}");
   verifyFormat("function x(): {x: string} {\n  return {x: 'x'};\n}");
   verifyFormat("function x(y: string): string {\n  return 'x';\n}");
   verifyFormat("for (var y: string in x) {\n  x();\n}");
   verifyFormat("for (var y: string of x) {\n  x();\n}");
   verifyFormat("function x(y: {a?: number;} = {}): number {\n"
                "  return 12;\n"
                "}");
+  verifyFormat("const x: Array<{a: number; b: string;}> = [];");
   verifyFormat("((a: string, b: number): string => a + b);");
   verifyFormat("var x: (y: number) => string;");
   verifyFormat("var x: P<string, (a: number) => string>;");
   verifyFormat("var x = {\n"
                "  y: function(): z {\n"
                "    return 1;\n"
                "  }\n"
                "};");