Introduce enum value for previously defined metadata -- make.implicit

Summary: This patch adds enum value for an existing metadata type -- make.implicit. Using preassigned enum will be helpful to get compile time type checking and avoid string construction and comparison. The patch also changes uses of make.implicit from string metadata to enum metadata. There is no functionality change.

Reviewers: reames

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D11698

llvm-svn: 243954