Let C++ inline functions return bool instead of sal_Bool

...to improve diagnosing misuses of boolean expressions in client code (cf.
compilerplugins/clang/implicitboolconversion.cxx).  This change should be
transparent to client code.

Missing overloads of insert() for bool have been added to OStringBuffer and
OUStringBuffer (which required dropping one !VALID_CONVERSION check that would
now pick that overload, but would be flagged by
compilerplugins/clang/pointertobool.cxx).

Change-Id: I2d64cd923b8f47bfaa31e753def6515c29a3f8c9