[Docs] Escape the @ symbol, so that it appears in documentation output. [NFC]

Summary:
The '@' character is a special character in Doxygen.  In a handful of cases we were not escaping this character which resulted in llvm intrinsics not being rendered properly.  Specifically, the @llvm part was removed.  

For example, see https://llvm.org/doxygen/classllvm_1_1AssumptionCache.html.  There are a few references to '.assume' without the @llvm. prefix.  This patch corrects this.

Reviewers: sanjoy

Reviewed By: sanjoy

Subscribers: llvm-commits, sanjoy

Differential Revision: https://reviews.llvm.org/D45981

llvm-svn: 330998