Do not swallow math comments in tex2lyx.

This is not needed, since LyX supports comments in math. Data loss with math
comments containing a backslash in LyX has been fixed as well.
The test case was found in bug #8104.

src/mathed/MathParser.cpp

@@ -1018,7 +1018,7 @@ bool Parser::parse1(InsetMathGrid & grid, unsigned flags,
 				Token const & t = getToken();
 				if (t.cat() == catNewline)
 					break;
-				s += t.asString();
+				s += t.asInput();
 			}
 			cell->push_back(MathAtom(new InsetMathComment(buf, s)));
 			skipSpaces();

src/tex2lyx/math.cpp

@@ -121,7 +121,7 @@ void parse_math(Parser & p, ostream & os, unsigned flags, const mode_type mode)
 
 		else if (t.cat() == catComment) {
 			if (!t.cs().empty())
-				cerr << "Ignoring comment: " << t.asInput();
+				os << t.asInput();
 			else
 				// "%\n" combination
 				p.skip_spaces();

src/tex2lyx/test/test-insets.tex

@@ -535,6 +535,15 @@ builtin \textasciicircum % with a comment
 A sub\textsubscript{sc\emph{ript}} and super\textsuperscript{script
 with $a^2+b^2=c^2$ math}.
 
+\section{Mathematics\index{Mathematics}}
+
+Let $f:\left[  a,b\right]  \rightarrow%
+%TCIMACRO{\U{211d} }%
+%BeginExpansion
+\mathbb{R}
+%EndExpansion
+$.
+
 \printindex{}
 
 \printnomenclature hello