graphics/openshadinglanguage: update to 1.11.15.0 release

Changelog:

• oslc/OSLCompiler: Check for errors when when writing oso (for example, disk volume full).
• Fix crash generating closure function calls with LLVM 11.
• Build: LLVM 12 compatibility.
• Build: Support for CMake 3.20
• Build: Be robust to certain OpenEXR 2.x config fies.
• Build: Fixes to deal with newer OIIO builds that are C++14.
• Fix library setup for LLVM installations consisting of static libraries.
• Fix warnings that occur when compiling with clang and C++17 mode.
• CI/testing: Additional CI tests for clang+c++17, gcc11, llvm 12, speed up CI builds by not building the testing components of OIIO.
• Changes for compatibility with OpenImageIO 2.3.
• Fix potential crashes (or at least undefined behavior) due to missing virtual destructor of internal Symbol type.

PR: 258440
Approved by: maintainer