[clang] NFC: remove superfluous braces

In commit 9bb33f57, a pair of superfluous braces are introduced to the function Sema::BuildDeclarationNameExpr.
This patch tries to remove the superfluous braces. Also use clang-format to further beautify the above function.

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D108609