Fix integer overflow in PCHReader when reading the length of an

identifier.  This caused a crash when reading PCH files that contained
long identifier names.

The issue is that 'StrLenPtr' was previously a 'const char *', meaning
the byte loaded from it would be interpretted as a signed integer.  If
the topmost bit was set, conversion to 'unsigned' would extend that
bit, causing an overflow.

The solution is to make 'StrLenPtr' an 'unsigned char *', always
treating the value as an unsigned integer.

This fixes: <rdar://problem/7328900>

llvm-svn: 84925

clang/lib/Frontend/PCHReader.cpp

@@ -2515,7 +2515,7 @@ IdentifierInfo *PCHReader::DecodeIdentifierInfo(unsigned ID) {
     // All of the strings in the PCH file are preceded by a 16-bit
     // length. Extract that 16-bit length to avoid having to execute
     // strlen().
-    const char *StrLenPtr = Str - 2;
+    const unsigned char *StrLenPtr = (const unsigned char*) Str - 2;
     unsigned StrLen = (((unsigned) StrLenPtr[0])
                        | (((unsigned) StrLenPtr[1]) << 8)) - 1;
     IdentifiersLoaded[ID - 1]