[NFC] Doc and style fixes for #filePath implementation

This addresses some late-breaking review comments left by @hamishknight on swiftlang#29412.

Author: beccadax

include/swift/AST/SourceFile.h

@@ -321,10 +321,13 @@ class SourceFile final : public FileUnit {
   /// forwarded on to IRGen.
   ASTStage_t ASTStage = Unprocessed;
 
-  /// Virtual filenames declared by #sourceLocation(file:) directives in this
-  /// file.
-  llvm::SmallVector<Located<StringRef>, 0> VirtualFilenames;
+  /// Virtual file paths declared by \c #sourceLocation(file:) declarations in
+  /// this source file.
+  llvm::SmallVector<Located<StringRef>, 0> VirtualFilePaths;
 
+  /// Returns information about the file paths used for diagnostics and magic
+  /// identifiers in this source file, including virtual filenames introduced by
+  /// \c #sourceLocation(file:) declarations.
   llvm::StringMap<SourceFilePathInfo> getInfoForUsedFilePaths() const;
 
   SourceFile(ModuleDecl &M, SourceFileKind K, Optional<unsigned> bufferID,

lib/AST/Module.cpp

@@ -1964,7 +1964,7 @@ SourceFile::getInfoForUsedFilePaths() const {
         getASTContext().SourceMgr.getLocForBufferStart(BufferID);
   }
 
-  for (auto &vpath : VirtualFilenames) {
+  for (auto &vpath : VirtualFilePaths) {
     result[vpath.Item].virtualFileLocs.insert(vpath.Loc);
   }
 

lib/Parse/ParseDecl.cpp

@@ -4827,7 +4827,7 @@ ParserStatus Parser::parseLineDirective(bool isLine) {
       if (!Filename.hasValue())
         return makeParserError();
       SourceLoc filenameLoc = consumeToken(tok::string_literal);
-      SF.VirtualFilenames.emplace_back(*Filename, filenameLoc);
+      SF.VirtualFilePaths.emplace_back(*Filename, filenameLoc);
 
       if (parseToken(tok::comma, diag::sourceLocation_expected, ",") ||
           parseSpecificIdentifier("line", diag::sourceLocation_expected,

lib/SILGen/SILGen.h

@@ -132,7 +132,7 @@ class LLVM_LIBRARY_VISIBILITY SILGenModule : public ASTVisitor<SILGenModule> {
   ASTContext &getASTContext() { return M.getASTContext(); }
 
   llvm::StringMap<std::pair<std::string, /*isWinner=*/bool>>
-  MagicFileStringsByFilePath;
+    MagicFileStringsByFilePath;
 
   static DeclName getMagicFunctionName(SILDeclRef ref);
   static DeclName getMagicFunctionName(DeclContext *dc);