test/lit.cfg: add a new substitution '%FileCheck'

'%FileCheck' removes absolute paths of the source and build directory
from the input.  Overwhelming majority of tests don't intend to match
these paths.

Also add a substitution '%raw-FileCheck' that does not sanitize the
input.

Author: gribozavr

docs/Testing.rst

@@ -358,6 +358,12 @@ Other substitutions:
 * ``%{python}``: run the same Python interpreter that's being used to run the
   current ``lit`` test.
 
+* ``FileCheck``: like the LLVM ``FileCheck`` utility, but occurrences of full
+  paths to the source and build directories in the input text are replaced with
+  path-independent constants.
+
+* ``%raw-FileCheck``: the LLVM ``FileCheck`` utility.
+
 When writing a test where output (or IR, SIL) depends on the bitness of the
 target CPU, use this pattern::
 

test/lit.cfg

@@ -998,6 +998,9 @@ config.substitutions.append(('%target-resilience-test', config.target_resilience
 config.substitutions.append(('%llvm-profdata', config.llvm_profdata))
 config.substitutions.append(('%llvm-cov', config.llvm_cov))
 
+config.substitutions.append(('\bFileCheck\b', 'sed -e \'s@%s@BUILD_DIR@\' -e \'s@%s@SOURCE_DIR@\' | FileCheck' % (swift_obj_root, config.swift_src_root)))
+config.substitutions.append(('%raw-FileCheck', 'FileCheck'))
+
 # If static stdlib is present, enable static stdlib tests
 static_stdlib_path = os.path.join(os.path.join(config.swift_lib_dir,"swift_static"), config.target_sdk_name)
 libswiftCore_path = os.path.join(static_stdlib_path,"libswiftCore.a")