clang-format: [JS] strict prop init annotation.

Summary:
TypeScript uses the `!` token for strict property initialization
assertions, as in:

    class X {
      strictPropAsserted!: string;
    }

Previously, clang-format would wrap between the `!` and the `:` for
overly long lines. This patch fixes that by generally preventing the
wrap in that location.

Reviewers: krasimir

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D48030

llvm-svn: 334415