Bump compiler-builtins to 0.1.15

This commit bumps the `compiler-builtins` dependency to 0.1.15 which
expects to have the source for `compiler-rt` provided externally if the
`c` feature is enabled. This then plumbs through the necessary support
in the build system to ensure that if the `llvm-project` directory is
checked out and present that we enable the `c` feature of
`compiler-builtins` and compile in all the C intrinsics.

1 files changed, 0 insertions, 0 deletions

diff --git a/src/llvm-project b/src/llvm-project
-Subproject 2c5656ae593851d0b2336a727cc14b77a06b8ac
+Subproject 4efebe31651d5520bcba968878dbb8a4971d204