Converted all method comments in IRBuilder.h to use doxygen style comments.

This implies changing method documentation from the following style:

/// MethodName - Method description...

to

/// \brief Method description...

ala the LLVM Style Guide.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172959 91177308-0d34-0410-b5e6-96231b3b80d8
Doxygenified Argument.h and performed some copy editing of the documentation.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172960 91177308-0d34-0410-b5e6-96231b3b80d8
Doxygenified and copy edited BasicBlock.h.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172961 91177308-0d34-0410-b5e6-96231b3b80d8