[torque] Add negative decimal literals.

This CL changes the DECIMAL_LITERAL lexer rule to allow
negative decimal literals as well.

This could also be achieved by using the unary minus operation,
which would occur an runtime overhead and feel counter-intuitive
for literals (imho).

R=tebbi@chromium.org

Change-Id: Ib01aa1930254bcd85a161de385b0fd4f176feb46
Reviewed-on: https://chromium-review.googlesource.com/1027473Reviewed-by: Daniel Clifford <danno@chromium.org>
Commit-Queue: Simon Zünd <szuend@google.com>
Cr-Commit-Position: refs/heads/master@{#52773}

src/torque/Torque.g4

@@ -107,9 +107,9 @@ fragment EXPONENT_PART
         ;
 
 DECIMAL_LITERAL
-        : DECIMAL_INTEGER_LITERAL '.' DECIMAL_DIGIT* EXPONENT_PART?
-        | '.' DECIMAL_DIGIT+ EXPONENT_PART?
-        | DECIMAL_INTEGER_LITERAL EXPONENT_PART?
+        : MINUS? DECIMAL_INTEGER_LITERAL '.' DECIMAL_DIGIT* EXPONENT_PART?
+        | MINUS? '.' DECIMAL_DIGIT+ EXPONENT_PART?
+        | MINUS? DECIMAL_INTEGER_LITERAL EXPONENT_PART?
         ;
 
 type : IDENTIFIER;