[flang] Flang relicensing changes for LLVM Apache 2.0 license

This changes the license information in many of the flang source files.
- Renamed LICENSE to LICENSE.txt.
- NVIDIA Copyright lines have been removed.
- Initial lines for files follow the LLVM coding convention (file name on the first line; Emacs mode information on the first line).
- License references have been replaced with the abridged LLVM text.
- License information was removed from the test files.
- No file header was placed on test files (these weren't in most LLVM test files).
- License information was added to documentation files where it was missing.

We did not add brief file summaries to the initial line.

See http://llvm.org/docs/DeveloperPolicy.html#new-llvm-project-license-framework
for a description of the new license.

See http://llvm.org/docs/CodingStandards.html#file-headers
for a description of the new LLVM standard file header.

Original-commit: flang-compiler/f18@add6cde7244b926ca457a1c74c21fea071b85329
Reviewed-on: https://github.com/flang-compiler/f18/pull/887