[Format] Add format check for throwing negative numbers

[jonathoma]

Summary

The code throw -1; is currently formatted by clang-format as
throw - 1;. This diff adds a fix for this edge case and a test to check
for this in the future.

For context, I am looking into a related bug in the clang-formatting of
coroutine keywords: co_yield -1; is also reformatted in this manner
as co_yield - 1;. A later diff will add these changes and tests for the
co_yield and co_return keywords.

clang/lib/Format/TokenAnnotator.cpp

TokenType determinePlusMinusCaretUsage(const FormatToken &Tok) {
 
     if (PrevToken->isOneOf(TT_CastRParen, TT_UnaryOperator))
       // This must be a sequence of leading unary operators.
       return TT_UnaryOperator;
 
     // Use heuristics to recognize unary operators.
     if (PrevToken->isOneOf(tok::equal, tok::l_paren, tok::comma, tok::l_square,
                            tok::question, tok::colon, tok::kw_return,
-                           tok::kw_case, tok::at, tok::l_brace))
+                           tok::kw_case, tok::at, tok::l_brace, tok::kw_throw))
       return TT_UnaryOperator;
 
     // There can't be two consecutive binary operators.
     if (PrevToken->is(TT_BinaryOperator))
       return TT_UnaryOperator;
 
     // Fall back to marking the token as binary operator.
     return TT_BinaryOperator;

clang/unittests/Format/FormatTest.cpp

TEST_F(FormatTest, UnderstandsUnaryOperators) {
   verifyFormat("a-- > b;");
   verifyFormat("b ? -a : c;");
   verifyFormat("n * sizeof char16;");
   verifyFormat("n * alignof char16;", getGoogleStyle());
   verifyFormat("sizeof(char);");
   verifyFormat("alignof(char);", getGoogleStyle());
 
   verifyFormat("return -1;");
+  verifyFormat("throw -1;");
   verifyFormat("switch (a) {\n"
                "case -1:\n"
                "  break;\n"
                "}");
   verifyFormat("#define X -1");
   verifyFormat("#define X -kConstant");
 
   verifyFormat("const NSPoint kBrowserFrameViewPatternOffset = {-5, +3};");