More doxygen/documentation cleanups.

This reduces the number of warnings generated by Doxygen by about 100
(roughly 10%).  Issues addressed:
(1) Primarily, backslash-escaped "@foo" and "#bah" in Doxygen comments
when they're not supposed to be Doxygen commands or links, and
similarly for "<baz>" when it's not intended as as HTML tag;
(2) Changed some \t commands (which don't exist) to \c ("to refer to a
word of code", as the Doxygen manual says);
(3) \precondition becomes \pre;
(4) When touching comments, deleted a couple of spurious spaces in them;
(5) Changed some \n and \r to \\n and \\r;
(6) Fixed one tiny typo: #pragms -> #pragma.

This patch touches documentation/comments only.

llvm-svn: 158422