Lower-case diagnostics in libsyntax and librustc

[brson]

Our convention for error messages is that the first letter should be lowercase; subsequent sentences in error massages begin with uppercase. Our diagnostics are not consistent in this though and there are a number that begin with capital letters. Find them and destroy them.

[mrshu]

I would like to take this one.

I am wondering if you would like to apply the same to llvm_err like here: https://github.com/mozilla/rust/blob/master/src/librustc/back/link.rs#L80 ?

[brson]

@mrshu Yes, llvm_err as well. Thanks!